Milk becomes spoiled because of the bacteria present in it. When milk is pasteurized, some but not all of the bacteria in it is killed, and the bacteria that remain eventually cause pasteurized milk to spoil.
All food contains bacteria, and raw milk contains an abundance of bacteria, some of which is harmful to humans. Along with the grass dairy cows graze upon, they consume microbes considered human pathogens. Milk in its raw state is a natural environment that promotes microbial growth. Among the microbes present in milk are some that cause diarrhea, vomiting, inflammatory bowel syndrome, consumption and fever. If milk is boiled sufficiently to kill all the bacteria and make it sterile, it loses a significant amount of its taste and nutritional value. Pasteurization, or heating it to a high temperature, kills some of the bacteria and retards the action of some enzymes so that the milk is not dangerous, stores longer than it otherwise would, but is still tasty and nutritious.
The U.S. Centers for Disease Control and Prevention recommends that people should never drink raw milk, as severe sickness and even death can result. Contrary to what some people believe, drinking raw milk has no special health benefits, and pasteurized milk is not the source of any diseases, allergies or other problems. Pasteurization merely kills disease-causing bacteria and causes milk to last longer.