I was thinking about time travel and realized there’s a problem I hadn’t considered before: if you go back in time 300 years, the viruses will be totally different from those of the present, so your body will have no immunity to them. You’ll probably get a cold or a flu then die. Time travelers will have to get vaccinations, just like modern American tourists traveling to India.
Then there’s the problem of people going back in time and bringing diseases into the present, either accidentally or on purpose. A trip to 1500 could bring back smallpox. Oops!
Do you know of any science fiction that explores this idea?