Warning: The following entry contains a link to some manparts. Shut up. You can see them on Wikipedia, too.
Up until now, we have covered diseases and disorders that, for the most part, you cannot get. You either have them or you don't from the day that you are born. If you don't already have increased aging or fused legs, good for you. You can induce hypertrichosis if you like, but that's your call, not mine.
This is unlike everything else that I have covered so far in that it can happen to anyone. It is not genetic. It is not induced by drugs. All you have to do is get bitten by the wrong mosquito to end up like this:
The Taiwanese woman above is currently dealing with elephantiasis. It is so named because, yes, the end result resembles an elephant's foot.
If you're a guy, this disease will also swell your scrotum in a way that a sketchy drug you ordered online might. Yeah, your junk is going to be ENORMOUS, but it will also be filled with worms. NOT IN THAT WAY, I mean actual worms.
Picture linked here is NSFW. If you don't like male genitals, don't look.
The scientific community is honestly more interested in what this disease does to a man's junk than what it does to anybody's leg. That's sad.
Most forms of elephantiasis are caused by roundworms of various species (Wuchereria bancrofti, Brugia malayi, and B. timori), to name a few. These live in pretty much every tropical-subtropical region in the known world (i.e. you are bound to find ONE of them in the area if you ever want a tropical getaway). The larvae travel by mosquito. Their destination is the human lymphatic system.
It is not entirely certain whether the swelling in the legs and genitals is caused by the worms themselves or by the human immune system's response to the worms. Whatever the case, the bulging is like a beacon to mosquitos that says "bite me." They take a few larvae out of one host and stick them in another while sucking some blood in the process. (If you think this sounds like the botfly, it sort of is, but the botfly is at least motile on its own in one stage of life.)
There is no cure, but damn if people are not trying with this one. They have put out antibiotics, an anti-worm drug called albendazole (which looks like something from Taco Bell, but is a potent vermicide), and started mapping the genome of one of the worms in an attempt to cure this. You hear a lot of talk about cancer, arthritis, etc., but although a million scientists are working on those, I have not seen such a fervent movement towards completely wiping out a pathogen since smallpox. These people not just trying to treat the symptoms - they want the worms dead, dead, dead.
Kill on - oh, wait, we can only see these things under a microscope.
Or you could, y'know, eradicate the people harboring the worms in those parts. They are a vital part of the creature's life cycle. The mosquitoes FAR outnumber the humans and will keep reproducing even if humans are not around, but the worm needs humans to work. You want to stop elephantiasis? Stop humans. (Yes, I realize how callous that sounds.)