Thinking about it, it seems that disease burden for Europeans got worse and worse with time. Some have said that the first farmers in the Middle East wouldn’t had any increase at all in infectious disease over foragers. That’s wrong, of course, because just being sedentary results in an increase in diarrheal diseases, worm burden, etc. Increased population density also caused trouble. Still, it wasn’t nearly as bad back then as it would be in later millennia.
I mentioned this a while ago, in the context of warfare: infectious disease wasn’t a huge problem for the legions. But the increase affected everyone, not just warriors.
It helps to think about critical community size (CCS). Consider a disease like measles, one that doesn’t last long and confers lifelong immunity. The virus needs fresh, never-infected hosts (we call them children) all the time, else it will go extinct. The critical community size for measles is probably more than half a million – which means that before agriculture, measles as we know it today couldn’t and didn’t exist. In fact, it looks as if split off from rinderpest within the last two thousand years. Mumps was around in Classical times (Hippocrates gives a good description), but it too has a large CCS and must be relatively new. Rubella can’t be ancient. Whooping cough has a smaller CCS, maybe only 100,000, but it too must postdate agriculture.
In Classical Greece, smallpox hadn’t arrived. It may have shown up in the Antonine plague (165-180 AD).
Some new infectious diseases didn’t last – like the English Sweate. But many came to stay.
Bubonic plague showed up in Justinian’s reign and killed off half the population. It didn’t persist then, but it hung around for centuries after the second outbreak in 1347.
Syphilis arrived in 1494. Typhus most likely also originated in the Americas. Cholera was old in parts of India, but only arrived in Europe during the 19th century (Russia in 1817, the rest of Europe in 1827).
Europeans must have been evolving resistance, but nature kept piling on. In some cases, like smallpox, fairly high virulence was a favored strategy for the pathogen -and nobody is very good at out-evolving a microorganism. Medicine was an ineffective pseudoscience, worse than useless.