system
Given the following post, respond as if you're commenting on Reddit:
human
Have diseases such as the bubonic plague, smallpox, and measles gotten less deadly over time?

I've heard with the Omicron variant, people were mentioning that it has become less deadly, which evolutionary speaking, helps it spread more.  I've also heard a similar account for Influenza - that the Spanish Flu became much more 'mild' (relatively speaking) simply because it was more 'fit' if it had a lower mortality rate.

I'm wondering how this relates with regards to other diseases.  For example, is this a factor in what made the successive black plague outbreaks less deadly (with the first one in the mid-14th century being infamously horrible)?  And not to downplay how bad smallpox or measles is today, but was there a time when it was even deadlier, and has since become less deadly?  Or do some diseases always stay at some mortality rate?

Edit: I should add I'm interested because my "common understanding" of why the black plague became less bad is acquired immunity.  But given this recent insight from COVID-19, I'm wondering if this "evolving to less deadly" was relevant for the black plague as well.  And as well for other diseases...
gpt
There’s a widespread misunderstanding that pathogens evolve to become less virulent. It’s not true; there are many counterexamples. This is a very well studied field for over 50 years; by far the major driver of pathogen evolution is transmission efficiency, but that is an immensely complex driver with tons of feedback from the host and environment. 

The point about host and environment also confuses the question of whether the pathogen actually is less virulent than its predecessors. With omicron, for example, you’re comparing the virulence of the original virus *in a completely non-immune population*, with omicron in a population that is very widely immune due to vaccination and prior infection.  

Similarly, with measles, you’re comparing the virulence of a virus in a 19th century population — crowded, poorly nourished, lacking vitamins, unsanitary — with a virus in the 20th century, with vitamin supplements, high-protein diets, and so on - even before the vaccine was widespread. 

So the general answer is that yes, sometimes pathogens become less virulent; sometimes they don’t; sometimes they look as if they do, even though they don’t. But there’s no general rule for this, it’s a spillover from actual drivers of selection, and external factors.