Could the eradication of smallpox have been a factor in the spread of HIV? That's the question posed by researchers in the journal BMC Immunology, who think that the vaccine might have offered partial protection against HIV. As smallpox was wiped out, fewer people received the vaccine. The HIV explosion followed. In this small study, the researchers exposed immune cells from 10 smallpox-vaccinated people to HIV. Cells from 10 people never vaccinated against smallpox were also exposed. And HIV did replicate much more successfully in the cells from the non-vaccinated subjects. Further research confirming the relationship between stopping the smallpox vaccine and the rise of HIV would not surprise William McNeill. The author of the classic book Plagues and Peoples also wrote a chapter titled "Patterns of Disease Emergence in History" for the 1993 book Emerging Viruses. He mused on our ability to "insulate ourselves from local and frequent disasters." But doing so comes at the cost of "creating a new vulnerability to some larger disaster." McNeill concluded: "Perhaps what we face as humans is a conservation of catastrophe."