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Inspired by this webcomic.
I understand that going back in time has the potential to expose you to a number of diseases that you have no resistance to (e.g.: the Bubonic Plague, Polio, Smallpox, etc.), but clearly this effect depends pretty heavily on exactly how far you go back in time.
Starting in modern times:
If you go back in time 10 years, you don't have to worry about disease (any more than usual), because the diseases present are roughly the same as the time you left.
If you go back in time 100 years, there are a few diseases you might encounter that could be a problem, like the Spanish Influenza or Smallpox. Many diseases though, you would be vaccinated against, even if they aren't common nowadays, like Polio and Whooping Cough.
If you go back in time 1000 years, you have a whole number of diseases that are you are not inoculated against, like the Bubonic Plague, Leprosy, and Smallpox. Even if you ignore those, pretty much every disease present at that time would be different enough from what you have encountered that your immune system is going to have to fight off a lot of minor diseases. See this post for more information.
If you go back far enough in time, say to the Paleolithic era, you no longer have to worry, because diseases don't spread very rapidly in hunter-gatherer environments. Any really deadly diseases would just wipe out one tribe and spread no further, meaning that all the disease around back then would be relatively benign.
If you go back even further, at some point the early hominids will be biologically different enough from you that any diseases that they have won't be able to effectively spread to you.
But what about the time in-between, around the time of the first civilizations? More broadly, would a time traveler to the Bronze Age--when humans were first really collecting in urban environments--have to worry about ancient diseases?