Every time they opened the doors to a spaceship, they “let out” some of the air. I will assume that the air is an invisible blob in space. I am not sure if it would disperse or clump together, but I would guess it would stay together. If that is the case, then it would stand to reason that there are blobs of earth air on the moon, in space, etc. Inside this air would be human-carried germs and the common cold, etc.

The sun’s radiation should kill off those things…. OR mutate them.

What if another inhabited planet has done the same thing? Maybe their air has come into our atmosphere. Maybe when they “visit” us, they open their ship’s doors and a flood of their air comes out, with viruses, and it is deadly to cows…

Oh, and… SUPERTAX IMPORTS!