There are many ways that food can become contaminated. Food microbes are present in the intestines of healthy animals that we raise as food. Other food can be contaminated during slaughter from having contact with feces or contaminated meat while being processed. Washing or being irrigated with water that has been contaminated can contaminate fruits and vegetables. Shellfish can be contaminated by bacteria that are naturally found in seawater or in sewage that has been dumped into the sea.
Food may also be contaminated in the final processing stage. Infected humans who handle the food may contaminate it. Cross contamination is also possible in the late stages of preparing, juice from other raw agricultural products. Kitchen utensils and surfaces can transfer microbes from food to food by not washing them after each use. Drippings from contaminated raw meat, fish or poultry can also contaminate thoroughly cooked food.
The way food is handled after preparation can also cause contamination. Given moist conditions and availability of nutrients, one bacterium quickly can produce billions in 12 hours. Even lightly contaminated food, left un- refrigerated overnight, can be highly infectious the following day. (Mead, 1999)