Think of the virus kinda like mildew - it doesn’t create the environment it needs, it’s just present enough around us that when it happens upon it, it can take advantage and reproduce. Humans create mucus and cough/sneeze to clear things out and keep things in the respiratory tract clear. Cold viruses just happen to be the ones that best spread around in that environment. And once it found its useful environment to live in, if some of the variations figured out how to increase that response, all the better for them. Viruses are all accidents, they find themselves in the right place to take advantage of our mechanisms and vulnerability. You body is trying to clear out mucus and “stuff” that’s blocking its function. The virus is just taking a ride along with it.

Ah!! Quick! He's getting to close to the secret rules that run the universe, like where the second sock in the dryer goes! He's on to us!

Joking aside, I both love that you asked this, and the responses that are posted. So much of the world we take at face value, "Because that's just how it is." I love the moment when a person decides to stop accepting it as a given and asks why a thing is. At worst the world becomes a little weirder, as the limits of human understanding become a little clearer, but often I've found you get a fun little bit of knowledge that hardly anyone bothers to go for.

Kudos!