[akiselev]

> Do the things that make viruses airborne also keep them from having a long gestation period and also highly lethal?

Surviving airborne before the advent of buildings meant surviving outside in direct sunlight with little in the way of protection from droplets or anything else. Organisms have a limited amount of abstract "evolutionary capital" and the costs, so to speak, of developing adaptations to survive airborne and evade an active immune system are astronomical. It's not impossible, but the two "features" work against each other during the development phase.