Yes, it's called evolution. But since this is hn, here're a few articles/papers on the subject.

https://evolution.berkeley.edu/evolibrary/news/071201_adenov...

https://www.pnas.org/content/pnas/early/2014/11/26/141333911...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2639916/pdf/896...

http://hedrick.ucsd.edu/BICD136/PDF-links/Galvani-Epid-evol....

I was asking specifically about this claim: “Viruses tend to become less deadly as they spread“

I understand the basics of evolution. I also don’t have any trouble understanding the trades between transmission and virulence. However, this is not the same as saying that the typical virus becomes less virulent over the short term during a major outbreak.