During production and preparation, food can become contaminated by pathogens (organisms that cause disease) in a variety of ways. Many foodborne microbes (microscopic organisms) are present in the intestines of animals raised for food, such as cows and chickens. Meat can become contaminated during slaughter if it comes into contact with intestines.

Fruits and vegetables can become contaminated if they are washed with water that is contaminated with animal manure or human sewage. Oysters and other filter-feeding shellfish can become contaminated with bacteria naturally present in sea water or with microbes present in human sewage dumped into the sea. Foods produced by the products of many individual animals (e.g., ground beef) can be readily contaminated because a pathogen from one animal can taint an entire batch of meat.

During food processing, food can become contaminated by infected humans who handle it. In the kitchen, microbes can be transferred between foods by knives, cutting boards or other utensils.

Food can also become contaminated after it is handled. Many types of bacterial microbes need to multiply before they reach levels that can cause illness. Bacteria multiply rapidly at temperatures between 40 and 140 degrees Fahrenheit (4 to 60 degrees Celsius). Therefore, foods contaminated with bacteria that are not refrigerated or cooked properly may cause illness. Food can also become contaminated after it is cooked if it comes into contact with other foods containing pathogens.

After microbes enter the body, symptoms appear after a period of time. This is called the incubation period, and it may last from hours to days depending on the type and quantity of microbes ingested. During the incubation period, the microbes move through the stomach to the intestine and attach to the intestinal walls, where they multiply. Some microbes remain in the intestine, others produce a toxin that enters the bloodstream and others penetrate deeper body tissues.

Some people are more susceptible to foodborne illnesses or may experience symptoms that are more severe. People who face a greater risk of developing foodborne illness include:

• Elderly people. As people age, their immune systems do not respond as quickly and effectively to infectious organisms.
  
  
• Infants and young children. The immune systems of very young people are not fully developed.
  
  
• People with weakened immune systems. People with certain conditions, such as the human immunodeficiency virus (HIV) or diabetes, have a greater risk of developing a foodborne illness.
  
  
• Pregnant women. Pregnant women have an increased risk of developing a foodborne illness after exposure to contaminated food.
  
  
• People exposed to a very high dose of a pathogen.