Problems with Time-Travel

One of the issues with time-travel that no work of fiction ever seems to cover, is the problem of epidemiology, virology and germ theory. Like, if someone went back to say, the Bronze age in Mesopotamia, or with the last pandemic we had, anytime even more recently like the 1960's to save JFK or the 1860's to save Lincoln, you'd be exposing a whole population to novel diseases they have no exposure or immunity to.

It's one of the things that takes me out of the element. Like, now I'm reading S.M. Stirling's 'Nantucket' series. I notice a lot of the Native Americans die off prematurely, being exposed to late 20th century diseases, like they would have in the OTL from smallpox, only much earlier. There's even a little anecdote about whole civilizations being wiped out, essentially because of exposure to mumps, which can make you sterile.

So, through the story, I'm like: "Why wouldn't this be global? Why wouldn't this happen with every population they encounter?" Of course, the writer has to take liberties of ignoring information in order to have a story. But realistically, disregarding the issues with physics and spacetime, if you could even go to the past, you would introduce an entirely new microscopic biome to what we are accustomed to, so it might be like an extinction-level event if it were even possible to practically do.

That's one of my biggest beefs with the feasibility of time-travel that I notice is overlooked.