The viruses that commonly cause food poisoning are found in water that has been contaminated with human bowel movements. The viruses invade foods such as oysters, clams, and other shellfish. If you drink the water or eat the seafood raw or partially cooked, you may become ill. Parasites can also cause food poisoning. The most common type of parasitic food poisoning is called trichinosis.
Trichinosis is caused by roundworms in pork.
Depending on the cause, symptoms may develop anywhere from hours to months after you eat contaminated food. The most common types of food poisoning produce symptoms within 30 minutes to 2 days. Some toxins in fish may take only a few minutes to cause symptoms. Food poisoning is often suspected when several people become ill after eating the same food. The doctor will try to determine what is causing your illness by asking about your symptoms and the kind of food you ate just before you became ill.