The first smallpox vaccine was in 1796, and was eradicated in 1980: if someone born in 1981 magically teleported to 1795, how safe would it be? What sort of timeframe would I be able to visit cities without fear of the plague?
Awful mixed prounouns aside: for those of you visiting this thread from the future, the FAQ has more information about health and disease.
Flu will get you pretty hard just about any time. The 1918 flu killed, depending on who you ask, a full 3% to 5% of the world's population. It also killed young healthy people, which is unusual for the flu. It normally is fatal just to the very old, very young or otherwise vulnerable.