Richard Preston’s Demon in the Freezer (2002) is a terrifying study of both the anthrax scare of 2001 and the exponentially greater danger of smallpox, the worst disease man has known.
There is no natural immunity to smallpox. And the vaccines that those among us old enough once received provided immunity for just five years.
Or to put it another way, if there is an outbreak of smallpox we may be relying on a vaccine that was invented in 1796.