system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does the body make SO MUCH snot during a cold?

The presence of snot makes sense I guess, but why does the body make so damn much of it? How could leaking enough fluid to damage surrounding skin and half-drown a person be productive?
gpt
What's important to understand about diseases is they're not creating symptoms because they're intelligent little fiends who enjoy your misery. Many of these symptoms have been evolutionarily selected-for to spread specifically that virus. Rhinoviruses for instance are adapted to live and thrive in your nose and usually can't find purchase at all in other biomes of the human body. So they make you sneeze, in the hope to get inhaled by others noses, and they make your nose run, in the hope the snot will get picked up and as people constantly touch their faces hitch a ride to a new nose. White Plague (tuberculosis) makes you cough because it lives in the lungs. Blue Plague (cholera) is adapted for your lower GI tract, hence it makes you poop all over the place. Malaria is adapted for your blood stream, hence it hitches a ride on bloodsucking mosquitos; lyme disease is also bloodborne, hence it uses ticks as its vector. Part of what makes the nastiest diseases so nasty is they are so virulent as to live anywhere in your body. Black Plague for instance comes in multiple horrifying varieties depending exactly on how it enters the body and where in its whimsy it chooses to colonize; Ebola is similar in that you're losing infected blood from every orifice and making new orifices to ooze blood/virus. COVID-19 on the other hand is relatively "nice" compared to Black Plague in that it can only get a foothold via the lungs or eyes, but like Black Plague once it gets in it gets everywhere. Not only can it kill you horribly, something like half of survivors have almost random debilitating symptoms for what may be the rest of their lives. Zoonotic diseases are like that, because by definition they've become so protean that not only can they colonize different biomes on the same species body, they can jump to different species.

Interestingly, diseases don't "want" to kill you, in that it means they die too. Admittedly their highest evolutionary priority is reproduction, but after time passes they have a tendency to evolve to be nonlethal. The simian version of HIV (SIV) for instance is endemic to wild primates but also doesn't appreciably affect their health. Meanwhile HIV in humans has killed 33 million people since the start of the HIV epidemic in 1981, and still kills something like a half million people a year.