There is nothing new except what has been forgotten.
—Marie Antoinette (1755–1793)
For the first hundred years or so, the story of vaccination was the story of smallpox. Smallpox first appeared in remote antiquity, perhaps 10 thousand years ago when humankind first embraced settled farming in preference to transhumance. It was, many say, the most feared of all ancient diseases: smallpox killed 20–30% of those who contracted it, disfiguring or blinding those that survived.
It has been suggested, primarily on the basis of extant historical evidence, that virulent smallpox did not appear in Europe until the early modern era – most probably during the seventeenth century – and gradually replaced an endemic and much less virulent form of the disease. This transition seems to have occurred during a series of erratic smallpox outbreaks during the fifteenth and sixteenth centuries. With more certainty we can say that smallpox's case fatality – the proportion of the infected population that dies – escalated over the centuries, peaking during the eighteenth century.
Homo sapiens is the only species susceptible to smallpox; there is no known animal reservoir for the disease. Smallpox is contagious – it is transmitted from person to person. The smallpox virus is usually transmitted via the respiratory tract, primarily by inhaling respiratory droplets. While contagious, smallpox is not highly transmissible; dense populations are necessary ...