There are two main causes of food poisoning: infectious agents and toxic agents. Infectious agents are most common and include viruses, bacteria, and parasites. Toxic agents include poisonous mushrooms, improperly prepared exotic foods (such as barracuda), or pesticides on fruits and vegetables.

Bad Bacteria

Some bacteria infect the intestines, causing inflammation and difficulty absorbing nutrients and water, leading to diarrhea. Other bacteria produce chemicals in foods (known as toxins) that are poisonous to the human digestive system. When eaten, these chemicals can lead to nausea and vomiting, kidney failure, and even death.

Salmonella causes bloody diarrhea in about 40,000 people each year. Ground beef, eggs, improperly pasteurized dairy products, undercooked pork, and poultry products are among the foods linked to salmonella outbreaks. The bacteria Campylobacter is also a common culprit in diarrheal illness and can cause symptoms such as diarrhea, cramping, abdominal pain, fever, nausea, and vomiting.

About 100 people die annually from salmonella, and it is usually linked back to raw or undercooked poultry meat. In fact, what most people don’t realize is that 80 to 100 percent of chickens in the United States carry this bacterium.

Dangerous Viruses

Norovirus and hepatitis A are viruses that have the ability to make you very ill. Hepatitis A can cause you to have a sudden fever, malaise, nausea, anorexia, and abdominal pain. It is usually followed by jaundice several days later. Hepatitis A can be linked back to anything from lunch meat, fruits and fruit juices, milk and milk products, vegetables, salads, shellfish, and even iced drinks.

People who are infected with either the norovirus or hepatitis virus and who work in food processing plants and restaurants can pass along the virus. Norovirus outbreaks are commonly linked back to shellfish, like raw clams and oysters, and salad ingredients.

Harmful Toxins

Most people have heard of poisonous mushrooms, and they can be a danger, causing symptoms as mild as a headache or as serious as death. There are other ways toxic food can make you sick. Some large game fish from tropical waters—like barracuda, grouper, snapper, and jacks can all cause ciguatera, a food poisoning produced by a particularly nasty marine algae. Symptoms like nausea, vomiting, muscle pain, and dizziness can last anywhere from a few days to a few years. Pesticides on unwashed produce can cause mild to severe illness with weakness, blurred vision, headache, cramps, diarrhea, increased production of saliva, and shaking of the arms and legs.

Pesky Parasites

Other organisms that can show up in your food and water supply and create havoc are parasites. Parasites are usually defined as organisms that obtain nourishment from another organism, without giving any benefit in return. In most industrialized countries, parasites are rarely a problem, but in developing countries, the problem can be serious.

Parasites usually latch on to the intestine and maintain their nutrition by absorbing partially digested food through the surface of their skin. In other words, they eat the food and nutrients designed for the host. They can cause abdominal discomfort, nervousness, fever, and weight loss.

Food-Borne Illness Diagnosis

Symptoms of a food-borne illness can include gas, bloating, mucus in the stools, chronic fatigue, weakness, insomnia, and itching in the anus or ears. It can be very difficult to get an accurate diagnosis with parasites. Intestinal parasites are often ruled out with a stool sample. A more sensitive test, the Enzyme-Linked ImmunoSorbent Assay or ELISA test, may be ordered. The best testing is done in dedicated parasitology labs, which are few and far between.

More than two thirds of people who are infected may have no signs or symptoms of illness, even though the parasite is living in their intestines. If you suspect you may have a parasite, especially if you have traveled overseas, been to a petting zoo, or you or your child are frequently in a high-contact setting like a nursing home or daycare center, then call your physician.

Preventing Parasites

Watch what you drink. Drink only from water supplies that have been approved by local health authorities. If your water comes from a well, have your water checked on a regular basis. When you go camping or hiking, bring your own water, and don’t drink from streams or rivers.

Wash raw fruits and vegetables well before you eat them. Wash your hands well before you cook food for yourself or for your family. Encourage your kids to wash their hands after every trip to the bathroom and especially before eating. Kids are inherently unsanitary, but if you can prevent them from eating dirt, it’s a good thing.

Be very cautious about eating wild game, as over 60 percent of deer tested positive for at least one type of parasite. Wild animals, ranging from bears to cougars will often eat other wild animals, such as mice that have fed on parasite-infected meat. Take great care about eating any raw or uncooked meat. Uncooked pork, for example, can host a number of different parasites, particularly toxoplasma infection. Undercooked lamb and beef can also cause toxoplasmosis, which can cause serious problems for newborns, the elderly, and those who are already immuno-compromised.