Viruses mutate over time and they could become more or less deadly over time.

The Spanish Flu mutated into a more deadly virus overtime.

https://www.history.com/news/spanish-flu-second-wave-resurge...

^ not a scientific article, simply to prove how bad the viewpoint of "mutate to be less deadly".

More reasonable article:

https://www.aap.com.au/yes-viruses-can-evolve-to-become-more...