Due to the evolution of bacteria with respect to our bodies natural defense mechanisms, traveling forward in time could kill you, and traveling back in time could kill everyone.

First off, why did you even bother replying to me in the first place if you agreed that not all infectious diseases can become plagues? I was making fun of the dozens of people who still believe that their bodies are blank slates that need to be conditioned to everything to even have a sliver of resistance.

Second, It's absolutely irreverent to the point whither or not it's a virus or a bacteria. The point is that a man from the past with a healthy body would still get the same symptoms as someone from now.

Third, no, a totally mundane non-lethal disease you've never been exposed too would still give you totally mundane non-lethal symptoms. Pathogens such as measles and smallpox swept though north America because they where very aggressive, and they where aggressive for a reason.

Both measles and smallpox originated from cows. Yes cows, classic plagues almost exclusively come from domesticated animals. Smallpox for a bull is nothing, but when it takes the leap to humans, it becomes deadly because it had evolved to infect a much more resilient host. Compared to the Americas, The eastern world was filled with animals that could be easily domesticated, that's why there where dozens of extremely deadly plagues that ravaged Europe for centuries, and over time they evolved a moderate resistance(Many cities relied on immigration to keep population in the positive due too high mortality)

It's for this very reason that the colonists didn’t bring back a plague from the Americas to ravaged though Europe. It's the same principle, the natives had many common illnesses that an easterner had never been exposed too, yet millions did not die back home.

I suggest reading the post further up by the actual microbiologist.

/r/Showerthoughts Thread Parent