Food may become contaminated as it is produced, or as it is prepared.

Animals

Many foodborne microbes are present in healthy animals (usually in their intestines) raised for food. Meat and poultry can become contaminated during slaughter by contact with small amounts of intestinal contents.

Fresh fruits and vegetables can be contaminated if washed or irrigated with water contaminated by animal manure or human sewage.

Food Handling

During food processing, foodborne microbes can be introduced by infected humans handling the food or by cross contamination with other raw agricultural products. For example, Shigella (bacterium) and Norwalk virus (Norovirus) can be introduced by an infected food handler's unwashed hands.

Food Preparation

In the kitchen, microbes can be transferred from one food to the other by using the same knife, cutting board, or other utensil without washing in between. Fully cooked foods can become re-contaminated if they touch other raw foods that contain pathogens.

Food Storage

Many bacterial microbes need to multiply to larger numbers in food in order to cause disease. Under warm and moist conditions, for instance, slightly contaminated food left out overnight can become highly infectious as bacterial microbes multiply. Prompt refrigeration or freezing prevents most bacteria on food from multiplying. High salt, sugar, or acid levels also keep bacteria from growing.

Heating Food

Microbes are killed by heat. If food is heated to an internal temperature above 160F (78C), even for a few seconds, this is enough to kill most bacteria, viruses, or parasites. Toxins produced by bacteria vary in their sensitivity to heat. For instance, the toxin that causes botulism is inactivated by boiling, whereas the staphylococcal toxin is not.