From what we've seen and heard regarding gain of function of the coronavirus in a lab, I think that it would warrant some consideration that there are other diseases and viruses out there that underwent the same research
What came to mind first was HIV and how it popped up out of no where ready to spread among the human population and if you know about certain groups' desire to pull the wool over people's eyes, then the "somebody had sexual intercourse with a monkey/chimp" seems like baloney. Not to mention that it doesn't seem to affect primates and I've never heard of it doing so (not to say it doesn't but that there's a suspicious lack of any examples ever given).
What other diseases - if any - were given gain of funtion besides the Coronavirus?
Other than that, the human can be trained to do things that cause natural diseases to have an even worse effect on them and their for their body to be unable to heal its self to the best of its ability