But living closely with animals and drinking their milk also presents risks, chief among them the increased likelihood that infections will jump from animals to people. Some of humanity’s nastiest scourges, including smallpox and measles, probably originated in domesticated animals. In the 19th century, as industrialization spurred urbanization and mass migration, milk became a major vector of disease. As late as 1938, illnesses from milk still accounted for one-quarter of all infectious diseases contracted from what people ate and drank that year. During this period, newly established health authorities began pushing for milk to be treated by heating it; this simple practice of pasteurizing milk would come to be considered one the great public-health triumphs of the modern era.