The presence of microorganisms in food is a natural and unavoidable occurrence. Cooking generally destroys most harmful bacteria, but undercooked foods, processed ready-to-eat foods, and minimally processed foods can contain harmful bacteria that are serious health threats. Meat, dairy, and poultry products are important reservoirs for many of the food-borne pathogens, including Salmonella, Campylobacter, Listeria, and Escherichia coli O157:H7. Animal by-products, such as feed supplements, may also transmit pathogens to food animals (for example, Salmonella and bovine spongiform encephalopathy). Seafood is another potential source of food-borne pathogens, such as Vibrio, Listeria, and Hepatitis A. Infectious doses of many of these pathogens are very low (~10 bacterial cells), increasing the vulnerability of the elderly, infants, and people with immunological deficiencies or organ transplants.