OK, question for you: What the hell actually IS mucus, and why do I suddenly have so damn much of it?

Seriously, you go through your life, no mucus to be seen, then suddenly you get sick and all this shit comes gushing out of you like waterfalls. It’s like your body, freaked out by unwanted intruders, decides to purge all of the moisture from your body through your nose so that the infection can’t get at it. The infection decides that if they can’t have it, no one can, so they spill toxic sludge into it. I mean, really, what the hell is that all about? Where does this stuff come from? How the hell do I make it go away???