Public health law courses typically focus a good deal of attention on two related topics: the duty of government agencies to control the spread of communicable diseases and their use of the police power to do so. While governments sometimes take forceful actions in responding to disease outbreaks, they can also act to prevent their occurrence. Indeed, one of the great triumphs of public health in the 20th century was the development of vaccines and their widespread use, which seemed on course to relegate many formerly crippling or deadly diseases to the history books. Particular success occurred with vaccinations against childhood diseases such as polio, smallpox, and measles, outbreaks of which once routinely closed schoolrooms, playgrounds, and community swimming pools. By the last quarter of the century, completion of an elaborate schedule of immunizations was not merely the standard in pediatric practice but an official requirement for school enrollment. As a result, the range of communicable diseases that had once terrified parents had become threats to be feared only in memory.