From Wikipedia:
Milk is an excellent medium for microbial growth,[14] and when stored at ambient temperature bacteria and other pathogens soon proliferate.[15]
The US Centers for Disease Control (CDC) says improperly handled raw milk is responsible for nearly three times more hospitalizations than any other food-borne disease source, making it one of the world’s most dangerous food products.[16][17] Diseases prevented by pasteurization can include tuberculosis, brucellosis, diphtheria, scarlet fever, and Q-fever; it also kills the harmful bacteria Salmonella, Listeria, Yersinia, Campylobacter, Staphylococcus aureus, and Escherichia coli O157:H7,[18][19] among others.