|Smallpox pustule gauge -- Edinburgh, Scotland, 1870-1930|
The English colonists tried to make the New World a haven from smallpox. Boston inspected all incoming ships; if one arrived with smallpox on board, it flew a quarantine flag and remained isolated until the disease had passed. People afflicted with smallpox were sent to stay — and often die — on Spectacle Island in Boston Harbor, safely distant from the city's population. However, even with these precautions, there was usually a smallpox outbreak every 12 years; each time, patients either survived and acquired immunity or they died.
An epidemic occurred "on schedule" in 1702 and then, for some unknown reason, not again in Boston for 19 years. As more and more children grew to maturity without being exposed to the pox, Bostonians knew that when the disease did return, it would be more devastating than ever before.
In the spring of 1721, a group of sailors brought smallpox with them when they came ashore in Boston. As soon as the first cases appeared, the town took dramatic measures to isolate the infected men, but it was too late. By May the city was in the grip of a virulent epidemic.
As noted by this article from the Boston Museum of Science:
Edward Jenner, an English country doctor and keen inoculator, later adapted the practice, developing a safer, more effective technique he called vaccination. Having noted that local people who caught cowpox gained immunity from the far more dangerous smallpox, he successfully induced such immunity in an experiment on a local boy, James Phipps, in 1796.