>... By the way, wasn't it a slave that actually had the idea of giving a sample of an infection to people to build up their immunity? I think his owner got the credit for it though.

Wasn't that something to do with an English milk maid who had cow-pox on her arms and stuff? She didn't suffer from smallpox or something. They gave a sample of her pox to some kid and he lived through smallpox. Bit vague the memory but essentially along those lines. :-)