Essentially it's this species of cloud-like organisms that don't like humans very much because of pollution and all our wars. So they release pathogens, although the exact type varies from cloud to cloud. One sprinkles viruses, another might shoot fungi or prions. I was thinking maybe they could be more centered in Asia because of the mountains, and Africa because of the deserts.
I know this idea has some flaws but i'm hoping with enough critique, I'll be able too make this Miasma scp make it.