system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do diseases only do bad things to us? Why are there no diseases/viruses that are actually GOOD for us?

Ladies and gentleman I don't need the old "Well it wouldn't be a disease if it was good for you" answers. That's obvious, but the spirit of the question is how come no one can cough in your face and give you increased strength for 2 days instead of making you feel ill?
gpt
Yes, there are good diseases.  For instance, retroviruses and bornaviruses account for 8-9% of the human genetic code, and the same is true for most mammals and birds.  One pretty useful thing retroviruses help with is the formation of the placenta and differentiation early in fetal development.  These genes have been proven to come from multiple strains of human endogenous retrovirus (herv), and suppressing them slows and hinders development and the ability to latch onto the uterine wall.  Just one example of a beneficial virus, and 8-9% of our DNA comes from them.

There are a surprising number of retrovirus genes that protect and aid the development of the fetus.  It is kinda strange, but it fascinates me.