This is a medical question of type : causes 
 What causes Foodborne Illnesses ? 
 The majority of foodborne illnesses are caused by harmful bacteria and viruses.2 Some parasites and chemicals also cause foodborne illnesses.
                
Bacteria
                
Bacteria are tiny organisms that can cause infections of the GI tract. Not all bacteria are harmful to humans.
                
Some harmful bacteria may already be present in foods when they are purchased. Raw foods including meat, poultry, fish and shellfish, eggs, unpasteurized milk and dairy products, and fresh produce often contain bacteria that cause foodborne illnesses. Bacteria can contaminate foodmaking it harmful to eatat any time during growth, harvesting or slaughter, processing, storage, and shipping.
                
Foods may also be contaminated with bacteria during food preparation in a restaurant or home kitchen. If food preparers do not thoroughly wash their hands, kitchen utensils, cutting boards, and other kitchen surfaces that come into contact with raw foods, cross-contaminationthe spread of bacteria from contaminated food to uncontaminated foodmay occur.
                
If hot food is not kept hot enough or cold food is not kept cold enough, bacteria may multiply. Bacteria multiply quickly when the temperature of food is between 40 and 140 degrees. Cold food should be kept below 40 degrees and hot food should be kept above 140 degrees. Bacteria multiply more slowly when food is refrigerated, and freezing food can further slow or even stop the spread of bacteria. However, bacteria in refrigerated or frozen foods become active again when food is brought to room temperature. Thoroughly cooking food kills bacteria.
                
Many types of bacteria cause foodborne illnesses. Examples include
                
- Salmonella, a bacterium found in many foods, including raw and undercooked meat, poultry, dairy products, and seafood. Salmonella may also be present on egg shells and inside eggs.  - Campylobacter jejuni (C. jejuni), found in raw or undercooked chicken and unpasteurized milk.  - Shigella, a bacterium spread from person to person. These bacteria are present in the stools of people who are infected. If people who are infected do not wash their hands thoroughly after using the bathroom, they can contaminate food that they handle or prepare. Water contaminated with infected stools can also contaminate produce in the field.  - Escherichia coli (E. coli), which includes several different strains, only a few of which cause illness in humans. E. coli O157:H7 is the strain that causes the most severe illness. Common sources of E. coli include raw or undercooked hamburger, unpasteurized fruit juices and milk, and fresh produce.  - Listeria monocytogenes (L. monocytogenes), which has been found in raw and undercooked meats, unpasteurized milk, soft cheeses, and ready-to-eat deli meats and hot dogs.  - Vibrio, a bacterium that may contaminate fish or shellfish.  - Clostridium botulinum (C. botulinum), a bacterium that may contaminate improperly canned foods and smoked and salted fish.
                
Viruses
                
Viruses are tiny capsules, much smaller than bacteria, that contain genetic material. Viruses cause infections that can lead to sickness. People can pass viruses to each other. Viruses are present in the stool or vomit of people who are infected. People who are infected with a virus may contaminate food and drinks, especially if they do not wash their hands thoroughly after using the bathroom.
                
Common sources of foodborne viruses include
                
- food prepared by a person infected with a virus  - shellfish from contaminated water  - produce irrigated with contaminated water
                
Common foodborne viruses include
                
- norovirus, which causes inflammation of the stomach and intestines  - hepatitis A, which causes inflammation of the liver
                
Parasites
                
Parasites are tiny organisms that live inside another organism. In developed countries such as the United States, parasitic infections are relatively rare.
                
Cryptosporidium parvum and Giardia intestinalis are parasites that are spread through water contaminated with the stools of people or animals who are infected. Foods that come into contact with contaminated water during growth or preparation can become contaminated with these parasites. Food preparers who are infected with these parasites can also contaminate foods if they do not thoroughly wash their hands after using the bathroom and before handling food.
                
Trichinella spiralis is a type of roundworm parasite. People may be infected with this parasite by consuming raw or undercooked pork or wild game.
                
Chemicals
                
Harmful chemicals that cause illness may contaminate foods such as
                
- fish or shellfish, which may feed on algae that produce toxins, leading to high concentrations of toxins in their bodies. Some types of fish, including tuna and mahi mahi, may be contaminated with bacteria that produce toxins if the fish are not properly refrigerated before they are cooked or served.  - certain types of wild mushrooms.  - unwashed fruits and vegetables that contain high concentrations of pesticides.