IGNORE the fuss over the news last week — the United Nations’ AIDS-fighting agency admits to overestimating the global epidemic by six million people. That was a sampling error, an epidemiologist’s Dewey Defeats Truman. Look instead at the fact that glares out from the Orwellian but necessary revision of the figures for earlier years. There it is, starkly: AIDS has peaked. New infections reached a high point in the late 1990’s — by the best estimate, in 1998. There must have been such moments in the past — perhaps A.D. 543, when Constantinople realized it would survive the Plague of...