Viruses are also linked to all sorts of nasty outcomes like Alzheimer’s, cancer, etc. I doubt we’ll have any means to knock transmission back a ton anytime soon but it’d be nice to do what we can.
With the Spanish flu in 1918 it took months for it to get around, mostly through rail and steam ships. With COVID-19 we saw some variants explode around the entire globe in a matter of _weeks_.
I spent a year on a rather isolated base on an island, with about 50 other people. Hygiene was certainly not as good as it is here in the civilised world as we often didn't have running water, ate wild meat that had simply been stored in the cold outside for days, food products with an expiration date a few years in the past, etc, and we just didn't get sick. We got sick when the supply ship came, though. It wasn't Antarctica either, the conditions are mostly similar to Northern Europe, not so different from Brittany.
I think the situation is quite similar to how humans lived for a long time, in small groups with only occasional contact with strangers, and in these conditions I'm sure infectious disease was much less common than it is now.
Of course disease did exist, but I think it would mostly have been diseases brought by insects or other vectors, not as much things like the flu or the common cold.
It's actually the exact opposite? Improved hygiene and vaccination means rates of illness have dropped.