Smallpox was one of the scourges of the 17th and 18th centuries. The list of symptoms is thoroughly pleasant – large pustules all over your skin that are more or less guaranteed to scar, terrible fever, and vomiting. (The disease is extinct now aside from samples kept in laboratories – yet another reason to be glad we live in this modern age.)
Inoculation for smallpox in America began sometime in the early 1720s in Boston, when a preacher named Cotton Mather found out about the idea from one of his slaves. the method was simple, if rather disgusting – you find someone who had a mild case of the disease, then grind up parts of their sores, and put it under the skin of the person being inoculated (generally in a scratch between the thumb and forefinger). The second person would get a less severe form of the disease, and afterward will be immune for about 10 years.
Statistically speaking, inoculation worked, but it wasn't without its drawbacks. Smallpox in any form was extremely dangerous, and people regularly died from being inoculated. also, because inoculating people was essentially infecting them with the disease there was always the chance that they could cause an outbreak. (A case of the cure being worse than the disease.)
It wasn't until 1796 that a less dangerous form of vaccination was introduced. A British doctor – of course he was British – named Edward Jenner discovered that being infected with cowpox (which was non-lethal) would give immunity to smallpox as well. Jenner had made this discovery by infecting a young boy with cowpox, and then trying to infect him with smallpox (in the name of science, I'm sure the child would have been delighted to know). Given his methods, it's not surprising Jenner's findings were frowned upon by the medical establishment, and cowpox-based vaccination didn't see wide use until sometime after 1800.