Did the Rise of Cities Help HIV Take Off?

If HIV had established itself in human beings hundreds of years ago — at least in its current lethal form — we would have known about it, and some of the millions of Africans forcibly taken into slavery would have carried the disease to the Americas. That didn't happen, and the disease didn't appear in people until the 20th century, with the first confirmed case found in blood drawn in 1959 from a man in Kinshasa.

What happened? Marx told McNeil that he believes, ironically, that a key event might have been the introduction of cheap syringes into Africa during campaigns to wipe out other infectious diseases during the middle of the 20th century. The syringes were often reused, and infected blood could have easily moved from one person to another — just as shared needles have spread HIV among drug users.

But Arizona's Worobey believes HIV likely emerged earlier in the 20th century, well before syringes were common in Africa. In this case, the difference might have been the growth of crowded cities in Africa. Before 1910 — around when Worobey believes the virus emerged — no town in Central Africa had more than 10,000 people. That has changed incredibly over the past century as Africans have moved from the countryside into cities — Lagos, the capital of Nigeria, has a metro population of more than 15 million, and it's growing 10 times faster than New York City. Megacities mean more people coming into contact with each other — and it means the chance for more red-light districts and risky behaviors of the sort that aren't easy to carry out in rural areas.