We depend primarily on dairy cattle and, to some extent, on goats for a supply of one of nature’s most nutritionally complete foods — milk. But milk can also be good food for bacteria. Raw, unpasteurized dairy products such as milk, cheese and yogurt can contain harmful bacteria, such as E. coli and Salmonella, as well as germs linked to dysentery and tuberculosis.

In pasteurization, a process named for the 19th-century scientist Louis Pasteur, milk is heated to a consistent temperature that kills harmful bacteria.

Pasteurization does not affect the nutritional value of milk. But it doesn’t kill all bacteria, so pasteurized dairy products must still be stored in the refrigerator to prevent spoilage.

Most dairy products sold within the United States have been pasteurized, a process that has been protecting people’s health for more than 120 years.