Milk is pasteurized by heating it to a temperature of about 62 Centigrade (about 143 Fahrenheit) and maintaining that temperature for half an hour. The treatment kills most of the bacteria, which cause milk to go sour quickly, or many produce disease in human beings.
This temperature is chosen it is the lowest temperature that will kill the micro bacterium tuberculosis, the carrier of tuberculosis which used to kill people of all ages in their millions. At the same time the temperature is low enough not to alter the taste and quality of the milk too much.
Pasteurized is therefore much safer than untreated milk, especially for babies, and in many countries is the only milk allowed to be sold. The process is named after the French scientist Louis Pesteur (1822-1895) whose discoveries revolutionized knowledge about the effects of bacteria.