Perhaps the world periodically needs an equivalent of the 1755 Lisbon earthquake, a chastening reminder that nature still has something to say about what human beings proudly, and prematurely, call “the conquest of nature.” The earthquake disturbed Europe’s Enlightenment serenity: Perhaps God has not really ordained a benevolently ordered universe. This should not have been news to Europe, which in the 14th century had lost more than half of its population to the Black Death plague, and had subsequently endured many lesser but nevertheless devastating epidemics.

In America, the first modern nation and the nation most committed to the modern project of taming nature’s capriciousness, the AIDS epidemic of the early 1980s was particularly traumatic. This was so even though the public-health threat from the disease was limited because the primary means by which it was transmitted were known risky behaviors involving sex or needles shared by drug users.

AIDS disabused Americans of their polio paradigm. The 1950s success of the Salk vaccine in removing the terror of polio had encouraged the belief that pharmacology could slay all infectious diseases.