Herd immunity can sometimes reduce mortality from a disease, but over the centuries has failed to end the curse of influenza, tuberculosis, smallpox, polio, rabies or dengue fever. It fits neatly, however, into a social Darwinist framework. Those who die are the “weak” — the poorest, the youngest and the oldest young — or can at any rate be classified as weak and deserving to die, because they died. Survival of the fittest requires discarding the weak. Remember the “let them die” hecklers who populated some of the 2011 Republican debates on health care.