[Caipirabob]

Could you even drink the water or survive exposure to the average human? We may have an idea of what the bacteria was like then, but I doubt our immune systems could adapt to the slight variations that would be inevitable over the millennium.

[who_would_fardels_bear]

Even worse potentially is that anyone who goes back in time harbors thousands of bacteria and viruses that no one in the past is immune to.

The time traveler ends up killing lots of people and eventually is hunted down and killed as well.