Other than grossing out your houseguests, what other problems would be faced by someone who never cleans the apartment, never changes the sheets, lets dirty laundry pile up, lets stuff start to grow in the shower and the fridge?
One category of problem is insect infestations. There are other categories also.
Clothes moth larvae need time to grow while eating your woolens etc, and they need undisturbed dark and a little humidity. Lots of insects want small cracks and crevices that are never vaccuumed, to reproduce in. Spiders will come to live off the insects and flies, but typically the human will whack visible spiders, preventing them from reaching fully effective numbers. If they are permitted, they will somewhat limit the numbers of other critters.
Flies love un-disposed-of garbage to eat and reproduce in, and flies spread certain diseases (hepatitis for instance). Bedbugs are almost unheard of since the invention of the washing machine, so they might not show up, but if they do you will itch. I mean, the person will itch.
It’s supply-side: if there are good conditions for creatures, creatures will appear.
Chiefly: lack of opportunity to pass on their genetic material.
Other possibilities: skin infections from skanky clothing, respiratory problems from mould spores (the black mould found in bathrooms is bad for this, I have heard), potential food poisoning from the old gunky food in the fridge; also lice/fleas from pets I imagine.