The most common cause of food poisoning is the ingestion of toxins produced by bacteria in contaminated food. Traveler's diarrhea can occur anywhere, but is more common in the underdeveloped world.

Bacteria that cause food poisoning include:

• E. coli:
• Toxigenic E. coli
• E. coli 0157:H7
• Shigella infection
• Salmonella infection
• Campylobacter enteritis
• Cryptosporidium gastroenteritis
• Staphylococcus aureus
• Vibrio cholera
• Listeria monocytogenes
• Vibrio parahaemolyticus (usually from ingesting shellfish)
• Clostridium perfringens:
• Food borne botulism
• Bacillus cereus Type 1

Toxigenic E. coli
Several types of E. coli bacteria that produce powerful toxins (chemicals) that can cause severe illness. Although E. coli is a normal intestinal bacteria, these bacteria have acquired (through evolution and the inter-species transfer of DNA) genes that allow them to produce dangerous toxins. Other toxigenic E. coli include: STEC, ETEC, and EHEC.

E. coli 0157:H7
E. coli 0157:H7 is a strain of E. coli bacteria that produces a powerful toxin that can cause severe illness. E. coli 0157:H7 is found in the intestines of healthy cattle, sheep, goats, and deer. Transmission to humans can occur when the stool of infected persons is passed from one person to another (if hygiene or hand washing habits are inadequate). Other routes of infection include: consumption of meat (particularly ground beef) that has not been cooked sufficiently to kill the bacteria and the consumption of contaminated sprouts, lettuce, or spinach. Drinking unpasteurized juice or milk can also transmit the bacteria. Some infections have developed after swimming in contaminated water.