BTW certain bugs will opportunistically take up residence in the...

BTW certain bugs will opportunistically take up residence in the elderly or immuno compromised, or for genetic reasons we don't fully understand.

Buruli ulcer is one example. A friend who is retirement age got it and it wasn't diagnosed for a very long time, and he nearly lost his foot.
Who gets this while the immune systems of others will resist is not fully understood. They've only recently accepted it's transmitted by mosquitoes probably biting infected possums beforehand.

Bird mites are an untested area. They're frustratingly tiny and if buried under skin can't be viewed by a microscope properly. The labs keep looking for something bigger. It's frustrating but got to stay calm.