Foodborne Illness

Foodborne illness is caused by consuming contaminated foods or beverages. Many different disease-causing germs, or pathogens, can contaminate foods, so there are many different foodborne infections. In addition, poisonous substances or toxins present in food can cause foodborne diseases.

Many microbes can spread in more than one way, so it isn't always clear whether a disease is foodborne. But finding out is important, because public health authorities need to know how a particular disease is spreading to take the appropriate steps to stop it. For example, Escherichia coli O157:H7 infections can spread through contaminated food, drinking water, or swimming water; and from toddler to toddler at a day care center. Depending on which type of spread caused a case, the measures to stop other cases from occurring could range from removing contaminated food from stores, chlorinating a swimming pool, or closing a child day care center.



Other Names

 * Contaminated food
 * Food poisoning

Types
Foodborne illness can be caused by either a live organism (a food infection) or by a toxin or other harmful substances (food poisoning or food intoxication). More than 250 different foodborne diseases have been described. Most of these diseases are infections, caused by a variety of organisms like bacteria, viruses, and parasites. Other diseases are true poisonings or intoxications, caused by harmful toxins or chemicals that are naturally present in or have contaminated the food&mdash;for example, poisonous mushrooms. These different diseases have many different symptoms, so there is no one syndrome of foodborne illness. However, the microbe or toxin enters the body through the gastrointestinal tract, and often causes the first symptoms there, so nausea, vomiting, abdominal cramps, and diarrhea are common symptoms in many foodborne diseases.

Signs and Symptoms
In most cases of foodborne illnesses, symptoms resemble "stomach flu" and may last from hours to days. Symptoms can range from mild to serious and include:


 * abdominal cramps
 * nausea
 * vomiting
 * diarrhea, which is sometimes bloody
 * fever
 * dehydration

After the pathogen or toxin is ingested, there is a delay, called the incubation period, before the symptoms of illness begin. This delay may range from hours to days, depending on the organism, and on how many of them were swallowed. During the incubation period, the microbes pass through the stomach into the intestine, attach to the cells lining the intestinal walls, and begin to multiply there. Some types of microbes stay in the intestine, some produce a toxin that is absorbed into the bloodstream, and some can directly invade the deeper body tissues. The symptoms depend on the microbe. Many organisms cause similar symptoms, especially diarrhea, abdominal cramps, and nausea. There is so much overlap that it is hardly ever possible to say which microbe is causing a given illness unless laboratory tests are done to identify the microbe, or unless the illness is part of a recognized outbreak.

Causes
Foodborne illness is caused by ingesting food or drink contaminated with a live pathogen or  a toxic substances (toxin). Organisms that can cause the disease or produce the toxin can be of several types.
 * Bacteria, such as Salmonella
 * Viruses, such as Norwalk virus
 * Parasites, such as Giardia
 * Fungal, such as toxic mushrooms
 * Algal, such as paralytic shellfish poisoning after algae blooms

Infections
The most commonly recognized foodborne infections are those caused by the bacteria Campylobacter, Salmonella, and E. coli O157:H7, and by a group of viruses called caliciviruses, also known as the Norwalk and Norwalk-like viruses.

Campylobacter is a bacterial pathogen that causes fever, diarrhea, and abdominal cramps. It is the most commonly identified bacterial cause of diarrheal illness in the world, causing a disease called campylobacteriosis. These bacteria live in the intestines of healthy birds, and most raw poultry meat has Campylobacter on it. Eating undercooked chicken, or other food that has been contaminated with juices dripping from raw chicken, is the most frequent source of this infection.

Salmonella is also a bacterium that is widespread in the intestines of birds, reptiles, and mammals. It can spread to humans via a variety of different foods of animal origin. The illness it causes, salmonellosis, typically includes fever, diarrhea, and abdominal cramps. In persons with poor underlying health or weakened immune systems, it can invade the bloodstream and cause life-threatening infections.

E. coli O157:H7 is a bacterial pathogen that lives in cattle and other similar animals. Human illness typically follows consumption of food or water that has been contaminated with microscopic amounts of cow feces. The illness it causes is often a severe and bloody diarrhea and painful abdominal cramps, without much fever. In 3% to 5% of cases, a complication called hemolytic uremic syndrome (HUS) can occur several weeks after the initial symptoms. This severe complication includes temporary anemia, profuse bleeding, and kidney failure.

Calicivirus, or Norwalk-like virus, is an extremely common cause of foodborne illness, though it is rarely confirmed by laboratory diagnosis, because the laboratory test is not widely available. It causes an acute gastrointestinal illness, usually with more vomiting than diarrhea, that resolves within two days. Unlike many foodborne pathogens that ordinarily reside in animals, it is believed that Norwalk-like viruses spread primarily from one infected human to another. Infected kitchen workers can contaminate a salad or sandwich as they prepare it, if they have the virus on their hands. Infected fishermen have contaminated oysters as they harvested them. In recent years, cruise ships have been struck by Norwalk-like virus outbreaks.

Some common diseases are occasionally foodborne, even though they are usually transmitted by other routes. These include infections caused by Shigella, hepatitis A, and the parasites Giardia lamblia and Cryptosporidia. Even strep throats have been transmitted occasionally through food.

Toxins
In addition to disease caused by direct infection, some foodborne diseases are caused by the presence of a toxin in the food that was produced by a microbe in the food. For example, the bacterium Staphylococcus aureus can grow in some foods and produce a toxin that causes intense vomiting. The rare but deadly disease botulism occurs when the bacterium Clostridium botulinum grows and produces a powerful paralytic toxin in foods. These toxins can produce illness even if the microbes that produced them are no longer there.

Poisons
Other toxins and poisonous chemicals can cause foodborne illness. People can become ill if a pesticide is inadvertently added to a food, or if naturally poisonous substances are used to prepare a meal. Every year, people become ill after mistaking poisonous mushrooms for safe species, or after eating poisonous reef fishes.

Food contamination
We live in a microbial world, and there are many opportunities for food to become contaminated as it is produced and prepared. Many foodborne microbes are present in healthy animals (usually in their intestines) raised for food. Meat and poultry carcasses can become contaminated during slaughter by contact with small amounts of intestinal contents. Similarly, fresh fruits and vegetables can be contaminated if they are washed or irrigated with water that is contaminated with animal manure or human sewage. Some types of Salmonella can infect a hen's ovary so that the internal contents of a normal-looking egg can be contaminated with Salmonella even before the shell is formed. Oysters and other filter feeding shellfish can concentrate Vibrio bacteria that are naturally present in sea water, or other microbes that are present in human sewage dumped into the sea.

Later in food processing, other foodborne germs can be introduced from infected humans who handle the food, or by cross-contamination from some other raw agricultural product. For example, Shigella bacteria, hepatitis A virus and Norwalk virus can be introduced by the unwashed hands of food handlers who are themselves infected. In the kitchen, microbes can be transferred from one food to another food by using the same knife, cutting board, or other utensil to prepare both without washing the surface or utensil in between. A food that is fully cooked can become recontaminated if it touches other raw foods or drippings from raw foods that contain pathogens.

The way that food is handled after it is contaminated can also make a difference in whether or not an outbreak occurs. Many bacteria need to multiply to a larger number before enough are present in food to cause disease. Given warm moist conditions and an ample supply of nutrients, one bacterium that reproduces by dividing itself every half-hour can produce 17 million progeny in 12 hours. As a result, lightly contaminated food left out overnight can be highly infectious by the next day. If the food were refrigerated promptly, the bacteria would not multiply at all. In general, refrigeration or freezing prevents virtually all bacteria from growing but generally preserves them in a state of suspended animation. This general rule has a few surprising exceptions. Two foodborne bacteria, Listeria monocytogenes and Yersinia enterocolitica can actually grow at refrigerator temperatures. High salt, high sugar, or high acid levels keep bacteria from growing, which is why salted meats, jam, and pickled vegetables are traditional preserved foods.

Microbes are killed by heat. Heating to an internal temperature above 160&deg;F, or 78&deg;C, for even a few seconds, is sufficient to kill parasites, viruses, and bacteria, except for the Clostridium bacteria, which produce a heat-resistant form called a spore. Clostridium spores are killed only at temperatures above boiling. This is why canned foods must be cooked to a high temperature under pressure as part of the canning process.

The toxins produced by bacteria vary in their sensitivity to heat. The staphylococcal toxin that causes vomiting is not inactivated even if it is boiled. Fortunately, the potent toxin that causes botulism is completely inactivated by boiling.

Diagnosis
Food infection is usually diagnosed by specific laboratory tests that identify the causative organism. Bacteria such as Campylobacter, Salmonella, and E. coli O157:H7 are found by culturing stool samples in the laboratory and identifying the bacteria that grow on the agar or other culture medium. Parasites can be identified by examining stools under the microscope. Viruses are more difficult to identify, as they are too small to see under a light microscope and are difficult to culture. Viruses are usually identified by testing stool samples for genetic markers that indicate a specific virus is present.

Many foodborne infections are not identified by routine laboratory tests, instead requiring specialized, experimental, and/or expensive tests that are not widely available. If the diagnosis is to be made, the patient has to seek medical attention, the physician must decide to order diagnostic tests, and the laboratory must use the appropriate procedures. Because many ill persons do not seek attention, and of those that do, many are not tested, many cases of foodborne illness go undiagnosed. For example, the CDC estimates that 38 cases of salmonellosis actually occur for every case that is actually diagnosed and reported to public health authorities.

Treatment
There are many different kinds of foodborne diseases and they may require different treatments, depending on the symptoms they cause. Illnesses that are primarily diarrhea or vomiting can lead to dehydration if the person loses more body fluids and salts (electrolytes) than they take in. Replacing the lost fluids and electrolytes and keeping up with fluid intake are important. If diarrhea is severe, oral rehydration solution such as Ceralyte, Pedialyte, or Oralyte, should be drunk to replace the fluid losses and prevent dehydration. Similar fluids have saved many lives in developing nations, where infectious diarrhea typically kills thousands of children every day. Sports drinks such as Gatorade do not replace the losses correctly and should not be used for the treatment of diarrheal illness. Preparations of bismuth subsalicylate (e.g., Pepto-Bismol) can reduce the duration and severity of simple diarrhea. If diarrhea and cramps occur, without bloody stools or fever, taking an antidiarrheal medication may provide symptomatic relief, but these medications should be avoided if there is high fever or blood in the stools because they may make the illness worse.

A health care provider should be seen for a diarrheal illness if the illness includes:
 * high fever (temperature over 101.5&deg;F, measured orally)
 * blood in the stools
 * repeatedly vomiting and being unable to keep liquids down (which can lead to dehydration)
 * signs of dehydration, including a decrease in urination, a dry mouth and throat, and feeling dizzy when standing up
 * diarrhea that lasts more than three days

Most food illnesses do not require treatment with an antibiotic. Many diarrheal illnesses are caused by viruses and will improve in two or three days without antibiotic therapy. In fact, antibiotics have no effect on viruses, and using an antibiotic to treat a viral infection could cause more harm than good. Even in the case of a mild bacterial infection, it is often not necessary to take an antibiotic. Other treatments can help the symptoms while the body fights off the infection, and careful handwashing can prevent the spread of infection to other people. Overuse of antibiotics is the principal reason many bacteria are becoming resistant. Resistant bacteria are no longer killed by the antibiotic. This means that it is important to use antibiotics only when they are really needed. An incomplete course of treatment can also cause bacteria to become resistant. If an antibiotic is prescribed, it is important to take all of the medication as prescribed, and not stop early just because symptoms seem to be improving.

Prevention
Raw foods of animal origin are the most likely to be contaminated; that is, raw meat and poultry, raw eggs, unpasteurized milk, and raw shellfish. Because filter-feeding shellfish strain microbes from the sea over many months, they are particularly likely to be contaminated if there are any pathogens in the seawater. Foods that mingle the products of many individual animals, such as bulk raw milk, pooled raw eggs, or ground beef, are particularly hazardous because a pathogen present in any one of the animals may contaminate the whole batch. A single hamburger may contain meat from hundreds of animals. A single restaurant omelet may contain eggs from hundreds of chickens. A glass of milk may contain milk from hundreds of cows. A broiler chicken carcass can be exposed to the drippings and juices of many thousands of other birds that went through the same cold water tank after slaughter.

Fruits and vegetables consumed raw are a particular concern. Washing can decrease but not eliminate contamination. Recently, a number of outbreaks have been traced to fresh fruits and vegetables that were processed under unsanitary conditions. These outbreaks show that the quality of the water used for washing and chilling the produce after it is harvested is critical. Using water that is not clean can contaminate many boxes of produce. Fresh manure used to fertilize vegetables can also contaminate them. Alfalfa sprouts and other raw sprouts pose a particular challenge, as the warm, moist conditions under which they are sprouted are ideal for growing microbes as well as sprouts, and because they are eaten without further cooking. That means that a few bacteria present on the seeds can grow to high numbers of pathogens on the sprouts. Unpasteurized fruit juice can also be contaminated if there are pathogens in or on the fruit that is used to make it.

A few simple precautions can reduce the risk of foodborne diseases:
 * Cook meat, poultry, and eggs thoroughly. Using a thermometer to measure the internal temperature of meat is a good way to be sure that it is cooked sufficiently to kill bacteria. For example, ground beef should be cooked to an internal temperature of 160&deg;F. Eggs should be cooked until the yolk is firm.


 * Don't cross-contaminate one food with another. Avoid cross-contaminating foods by washing hands, utensils, and cutting boards after they have been in contact with raw meat or poultry and before they touch another food. Put cooked meat on a clean platter, rather back on one that held the raw meat.


 * Refrigerate leftovers promptly. Bacteria can grow quickly at room temperature, so refrigerate leftover foods if they are not going to be eaten within four hours. Large volumes of food will cool more quickly if they are divided into several shallow containers for refrigeration.


 * Wash produce. Rinse fresh fruits and vegetables in running tap water to remove visible dirt and grime. Remove and discard the outermost leaves of a head of lettuce or cabbage. Because bacteria can grow well on the cut surface of fruit or vegetable, be careful not to contaminate these foods while slicing them up on the cutting board, and avoid leaving cut produce at room temperature for many hours.


 * Avoid becoming a source of foodborne illness. Wash hands with soap and water before preparing food. Avoid preparing food for others while sick with a diarrheal illness. Changing a baby's diaper while preparing food is a bad idea that can easily spread illness.

By starting at the store with safe food handling practice, consumers can play a major role in preventing foodborne illness. In this new Consumer Update video, the U.S. Food and Drug Administration (FDA) provides tips for keeping food safe while guiding viewers through a grocery store.

Safe cooking temperatures table
Suspected foodborne illnesses should be reported to a local health department, an important part of the food safety system. Often calls from concerned citizens are how outbreaks are first detected. In public health investigations, it can be as important to talk to healthy people as to ill people. Cooperation may be needed even from those who are not ill.

People can protect themselves from foodborne disease when ordering food in a restaurant, just as they would at home. When ordering a hamburger, one can ask for it to be cooked to a temperature of 160oF and send it back if it is still pink in the middle. Before ordering something that is made with many eggs pooled together, such as scrambled eggs, omelets, or French toast, one can ask the waiter whether it was made with pasteurized egg, and choose something else if it was not.

Making food safe in the first place is a major effort, involving the farm and fishery, the production plant or factory, and many other points from the farm to the table. Many different groups in public health, industry, regulatory agencies, and academia have roles to play in making the food supply less contaminated. Consumers can vote for food safety with their dollars, by purchasing foods that have been processed for safety. For example, milk pasteurization was a major advance in food safety that was developed by Louis Pasteur in the late 19th century. Buying pasteurized milk rather than raw unpasteurized milk still prevents an enormous number of foodborne diseases every day. Now juice pasteurization is a recent important step forward that prevents E. coli O157:H7 infections and many other diseases. Consumers can look for and buy pasteurized fruit juices and ciders. In the future, meat and other foods will be available that have been treated for safety with irradiation.

Foodborne diseases are largely preventable, though there is no simple one-step prevention measure like a vaccine. Instead, measures are needed to prevent or limit contamination all the way from farm to table. A variety of good agricultural and manufacturing practices can reduce the spread of microbes among animals and prevent the contamination of foods.

For some particularly risky foods, even the most careful hygiene and sanitation are not enough to prevent contamination, and a definitive microbe-killing step must be included in the process. For example, early in the century, large botulism outbreaks occurred when canned foods were cooked insufficiently to kill the botulism spores. After research was done to find out exactly how much heat was needed to kill the spores, the canning industry and the government regulators went to great lengths to be sure every can was sufficiently cooked. As a result, botulism from commercial canned foods disappeared in the United States. Similarly the introduction of careful pasteurization of milk eliminated a large number of milk-borne diseases. This occurred after sanitation in dairies had already reached a high level.

Food irradiation
In the future, other foods can be made much safer by new pasteurizing technologies, such as in-shell pasteurization of eggs, and irradiation of ground beef. Just as with milk, these new technologies are ideally implemented in addition to good sanitation, not as a replacement for it.

Food irradiation is the treatment of food with high energy such as gamma rays, electron beams, or x-rays as a means of cold pasteurization, which destroys living bacteria to control foodborne illnesses. The United States relies exclusively on the use of gamma rays, which are similar to ultraviolet light and microwaves and pass through food, leaving no residue. Food irradiation is approved for wheat, potatoes, spices, seasonings, pork, poultry, red meats, whole fresh fruits, and dry or dehydrated products. Although irradiation destroys many bacteria, it does not sterilize food. Irradiated food still must be safely handled to safeguard against any surviving organisms. While traveling with food, perishable items such as meats should be wrapped to prevent leakage. Coolers should be filled with plenty of ice and stored in the car, not the trunk. If any food seems warmer than 40°F, it should be thrown out.

Chances of Developing
According theWorld Health Organization (WHO):
 * In 2005 alone, 1.8 million people died from diarrhea. A great proportion of these cases can be attributed to contamination of food and drinking water. Additionally, diarrhea is a major cause of malnutrition in infants and young children.
 * In industrialized countries, the percentage of the population suffering from foodborne diseases each year has been reported to be up to 30%. Many of these diseases cause a few days' missed school or work and go unreported, but many others cause more serious illness.
 * While less well-documented, developing countries bear the brunt of the problem due to the presence of a wide range of foodborne diseases, including those caused by parasites. The high prevalence of diarrheal diseases in many developing countries suggests major underlying food safety problems.
 * While most foodborne diseases are sporadic and often not reported, foodborne disease outbreaks may take on massive proportions. For example, in 1994, an outbreak of salmonellosis due to contaminated ice cream occurred in the U.S., affecting an estimated 224,000 people. In 1988, an outbreak of hepatitis A from the consumption of contaminated clams affected some 300,000 people in China.
 * Tthe CDC estimates that foodborne illness in the U.S. causes 76 million illnesses, 325,000 hospitalizations, and 5,200 deaths each year. The specific pathogens are identified for an estimated 14 million illnesses, 60,000 hospitalizations, and 1,800 deaths annually.

Risk factors
Some people are at particularly high risk if they get a foodborne illness, and should take extra precautions.
 * Pregnant women, the elderly, and those with weakened immune systems are at higher risk for severe infections such as Listeria and should be particularly careful not to consume undercooked animal products. They should avoid soft French-style cheeses, pates, uncooked hot dogs, and sliced deli meats, which have been sources of Listeria infections. Persons at high risk should also avoid alfalfa sprouts and unpasteurized juices.[[image: 7909_pregnant.jpg|thumb| In order to avoid acquiring a possible foodborne illness from eating a peeled food, one should wash such a food in warm water prior to peeling, which will greatly reduce the chances of introducing a foodborne pathogen to the clean interior. Pregnant women are at particular risk of foodborne illness. Source: CDC. James Gathany]]


 * A bottle-fed infant is at higher risk for severe infections with Salmonella or other bacteria that can grow in a bottle of warm formula if it is left at room temperature for many hours. Particular care is needed to be sure the baby's bottle is cleaned and disinfected and that leftover milk, formula, or juice is not held in the bottle for many hours.


 * Persons with liver disease are susceptible to infections with a rare but dangerous microbe called Vibrio vulnificus, found in oysters. They should avoid eating raw oysters.

Complications
In the last 15 years, several important diseases of unknown cause have turned out to be complications of foodborne infections. For example, we now know that Guillain-Barre syndrome can be caused by Campylobacter infection, and that the most common cause of acute kidney failure in children, hemolytic uremic syndrome, is caused by infection with E. coli O157:H7 and related bacteria. In the future, other diseases whose origins are currently unknown may turn out be related to foodborne infections.

Research
As new foodborne diseases emerge, several questions need to be answered before the problem can be successfully controlled. It takes careful scientific observation and research to answer these questions. Some pressing unanswered questions include:


 * How do the foodborne germs spread among the animals themselves, and how can this be prevented? This includes E. coli O157:H7 among cattle,  Salmonella among egg-laying hens, and Campylobacter in broiler chickens. If we could prevent the animals from becoming infected in the first place, we could cut down on cases of illness in the humans who eat them.
 * What is the microbial cause of outbreaks in which no pathogen can be identified by current methods? This is true for over half of the reported foodborne outbreaks.  Will it help to more frequently use the tests we already have, or are these outbreaks caused by pathogens we simply do not yet know how to identify?
 * What would be the effect of basic food-safety education of restaurant workers on the risk of foodborne disease among restaurant patrons?
 * How can the food and water that animals consume be made safer?
 * How can we dispose of animal manure usefully, without threatening the food supply and the environment?
 * How should basic food-safety principles best be taught to school children?
 * How can we be sure food-safety standards in other countries are as good as those in the United States? As we import more of our fresh foods from other countries, we need to be confident that they are produced with the same level of safety as food in the United States. Recent headlines concerning contaminated food from China  underscore the importance  of safe food production in our globalized economy.
 * What control strategies in the slaughter plant will reduce the contamination of poultry meat with Campylobacter?
 * How do raspberries in Central America get contaminated with Cyclospora in the first place? Does this parasite have an animal reservoir?
 * How can alfalfa sprouts and other raw sprouts be produced safely? Sprouts are unique among foods in that the conditions for sprouting are also perfect for bacterial growth, and they are not cooked after that.
 * How can irradiation of certain high-risk foods, such as ground beef, be used most effectively?

Controversy
Some organizations disagree with the necessity of pasteurization and irradiation for food safety. An argument against pasteurization is found here. The Organic Consumers Organization opposes food irradiation.

In addition, some believe that raw and "live" food is healthier.

Scathing critiques of the food industry, such as Eric Schlosser's Fast Food Nation, have blamed cases of foodborne illness on the very way in which factory farms are set up.

Epidemiology
An estimated 76 million cases of foodborne disease occur each year in the United States. The great majority of these cases are mild and cause symptoms for only a day or two. Some cases are more serious, and the CDC estimates that there are 325,000 hospitalizations and 5,000 deaths related to foodborne diseases each year. The most severe cases tend to occur in the very old, the very young, those who have an illness that reduces their immune system function, and healthy people exposed to a very high dose of an organism.

Routine monitoring of important diseases by public health departments is called disease surveillance. Each state decides which diseases are to be under surveillance in that state. In most states, diagnosed cases of salmonellosis, E. coli O157:H7, and other serious infections are routinely reported to the health department. The county reports them to the state health department, which reports them to CDC. Tens of thousands of cases of these "notifiable conditions" are reported every year. Each year, nearly 40,000 cases of salmonellosis are reported to the CDC. However, most foodborne infections go undiagnosed and unreported, either because the ill person does not see a doctor, or the doctor does not make a specific diagnosis. Also, infections with many pathogens are not reportable in the first place.

To get more information about infections that might be diagnosed but not reported, CDC developed a special surveillance system called FoodNet. FoodNet provides the best available information about specific foodborne infections in the United States, and summarizes them in an annual report.

In addition to tracking the number of reported cases of individual infections, states also collect information about foodborne outbreaks, and report a summary of that information to the CDC. About 400–500 foodborne outbreaks investigated by local and state health departments are reported each year. This includes information about many diseases that are not notifiable and thus are not under individual surveillance, so it provides some useful general information about foodborne diseases.

Outbreaks
An outbreak of foodborne illness occurs when a group of people consume the same contaminated food and two or more of them come down with the same illness. It may be a group that ate a meal together somewhere, or it may be a group of people who do not know each other at all, but who all happened to buy and eat the same contaminated item from a grocery store or restaurant. For an outbreak to occur, something must have happened to contaminate a batch of food that was eaten by a the group of people. Often, a combination of events contributes to the outbreak. A contaminated food may be left out at room temperature for many hours, allowing the bacteria to multiply to high numbers, and then be insufficiently cooked to kill the bacteria.

Investigating outbreaks
A foodborne outbreak is an indication that something needs to be improved in a food safety system. Public health scientists investigate outbreaks to control them, and also to learn how similar outbreaks can be prevented in the future. Just as when a fire breaks out in a large building or when an airliner crashes, two activities are critical when an outbreak occurs. First, emergency action is needed to keep the immediate danger from spreading, and, second, a detailed objective scientific investigation is needed to learn what went wrong, so that future similar events can be prevented. Much of what we know about foodborne disease and its prevention comes from detailed investigation of outbreaks. This is often how a new pathogen is identified, and this is how the critical information linking a pathogen to a specific food and animal reservoir is first gathered. The full investigation can require a team with multiple talents, including the epidemiologist, microbiologist, food sanitarian, food scientist, veterinarian, and factory process engineer.

The initial clue that an outbreak is occurring can come in various ways. It may be when a person realizes that several other people who were all together at an event have become ill and he or she calls the local health department. It may be when a physician realizes she has seen more than the usual number of patients with the same illness. It may be when a county health department gets an unusually large number of reports of illness. The hardest outbreaks to detect are those that are spread over a large geographic area, with only a few cases in each state. These outbreaks can be detected by combining surveillance reports at the regional or national level and looking for increases in infections of a specific type. This is why state public health laboratories determine the serotype (strain) of Salmonella bacteria isolated from people. New "DNA fingerprinting" technologies can make detecting outbreaks easier too. For example, the new molecular subtyping network, PulseNet, allows state laboratories and the CDC to compare strains of E. coli O157:H7 and an increasing number of other pathogens from all across the United States to detect widespread outbreaks.

After an apparent cluster of cases is detected, it is important to determine whether these cases represent a real increase above the expected number of cases and whether they really might be related. Sometimes a cluster of reported cases is caused by something other than an actual outbreak of illness. For example, if the person responsible for reporting has just returned from a vacation and is clearing up a backlog of cases by reporting them all at once, the sudden surge of reports is just a false cluster.

Once an outbreak is strongly suspected, an investigation begins. A search is made for more cases among persons who may have been exposed. The symptoms and time of onset, and location of possible cases is determined, and a "case definition" is developed that describes these typical cases. The outbreak is systematically described by time, place, and person. A graph is drawn of the number of people who fell ill on each successive day to show pictorally when it occurred. A map of where the ill people live, work, or eat may be helpful to show where it occurred. Calculating the distribution of cases by age and sex shows who is affected. If the germ is not known, samples of stool or blood are collected from ill people and sent to the public health laboratory to make the diagnosis.

To identify the food or other source of the outbreak, the investigators first interview a few persons with the most typical cases about exposures they may have had in the few days before they got sick. In this way, certain potential exposures may be excluded while others that are mentioned repeatedly emerge as possibilities. Combined with other information, such as the likely sources for the specific microbe involved, these hypotheses are then tested in a formal epidemiologic investigation. The investigators conduct interviews about a list of possible exposures with the ill persons, and with a comparable group people who are not ill. By comparing how often an exposure is reported by ill people and by well people, investigators can measure the association of the exposure with illness. Using probability statistics, similar to those used to describe coin flips, the probability of no association is directly calculated.

For example, imagine that an outbreak has occurred after a catered event. Initial investigation suggested that Hollandaise sauce was eaten by at least some of the attendees, so it is on the list of possible hypotheses. Now, we interview 20 persons who attended the affair, 10 of whom became ill and 10 who remained well. Each ill or well person is interviewed about whether or not they ate the Hollandaise sauce, as well as various other food items. If half the people ate the sauce, but the sauce was not associated with the illness, then we would expect each person to have a 50/50 chance of reporting that they ate it, regardless of whether they were ill or not. Suppose, however, that we find that all 10 ill people but none of the well persons reported eating Hollandaise sauce at the event? This would be very unlikely to occur by chance alone if eating the Hollandaise sauce were not somehow related to the risk of illness. In fact, it would be about as unlikely as getting heads ten times in a row by flipping a coin (that is 50% multiplied by itself 10 times over, or a chance of just under 1 in 1000). So the epidemiologist concludes that eating the Hollandaise sauce was very likely to be associated with the risk of illness. Note that the investigator can draw this conclusion even though there is no Hollandaise sauce left to test in a laboratory. The association is even stronger if she can show that those who ate second helpings of Hollandaise were even more likely to become ill, or that persons who ate leftover Hollandaise sauce that went home in doggie bags also became ill.

Once a food item is statistically implicated in this manner, further investigation into its ingredients and preparation, and microbiologic culture of leftover ingredients or the food itself (if available) may provide additional information about the nature of contamination. Perhaps the Hollandaise sauce was made using raw eggs. The source of the raw eggs can be determined, and it may even be possible to trace them back to the farm and show that chickens on the farm are carrying the same strain of Salmonella in their ovaries. If so, the eggs from that farm can be pasteurized to prevent them from causing other outbreaks.

Some might think that the best investigation method would be just to culture all the leftover foods in the kitchen, and conclude that the one that is positive is the one that caused the outbreak. The trouble is that this can be misleading, because it happens after the fact. What if the Hollandaise sauce is all gone, but the spoon that was in the sauce got placed in potato salad that was not served at the function? Now, cultures of the potato salad yield a pathogen, and the unwary tester might call that the source of the outbreak, even though the potato salad had nothing to do with it. This means that laboratory testing without epidemiologic investigation can lead to the wrong conclusion.

Even without isolating microbes from food, a well-conducted epidemiologic investigation can guide immediate efforts to control the outbreak. A strong and consistent statistical association between illness and a particular food item that explains the distribution of the outbreak in time, place, and person should be acted upon immediately to stop further illness from occurring.

An outbreak ends when the critical exposure stops. This may happen because all the contaminated food is eaten or recalled, because a restaurant is closed or a food processor shuts down or changes its procedures, or an infected food handler is no longer infectious or is no longer working with food. An investigation that clarifies the nature and mechanism of contamination can provide critical information even if the outbreak is over. Understanding the contamination event well enough to prevent it can guide the decision to resume usual operations, and lead to more general prevention measures that reduce the risk of similar outbreaks happening elsewhere.

Books
The following are nonfiction books that examine the food safety system and its shortcomings in the United States.


 * Eric Schlosser. Fast Food Nation. 2001. This book was made into a film in 2006.
 * Marion Nestle. Food Politics.
 * Nicols Fox. Spoiled: Why Our Food Is Making Us Sick, and It Was Probably Something You Ate.