Disclaimer: I do not own the rights to Ranma ½. The trademark and characters belong to Rumiko Takahashi and VIZ Communications. I'm just borrowing them to write fan fiction.
Jusenkyo
Mention the name Jusenkyo and most of the world's population would have a blank look on their face and say 'HUH?'. For people that live near the cursed valley, it is met with shudders and looks of fear. For the few that have actually set foot in the valley, it means a lifetime of horror and despair.
The valley itself is nigh inaccessible in the Bayankala mountains in a remote region of Western China in the Qinhai province. An underground river provides the water for the numerous springs found in the valley. Each spring has a pool that is large enough for a person to swim a few strokes and deep enough to be over their head. Hundreds of springs. Such a site would surely be ideal for a spa. If it wasn't for the effect the waters would have on the visitors.
The locals shun the valley. They shun even the waters, especially the waters. Local lore spanning generations tell of what happens to anyone exposed to the water of a spring. It changes them. A lucky few stay human, sort of. The majority of exposed people turn into an animal. Depending on the pool the water comes from, they can literally be any animal from A to Z. They keep their humanity and intelligence while in their cursed form. There is no known permanent cure and only a temporary means of returning to their original form. As the curse was created by water someone in the distant past was smart enough to find a way of reversing the curse.
Hot water. Pretty simple when you think about it. Cold spring water caused the curse so hot spring water should reverse it. People cursed were brought back and everyone thought they were cured. Until they were splashed with ordinary cold water and changed to their cursed form. Ordinary hot water returned them to human. Many went mad and took their lives in despair. Others thought they could find a spring and cancel out the curse. To their horror the curses combined, creating horrible creatures that were usually slain before anyone could determine who they were.
Some people were lucky when they were cursed by falling into one of the few pools that allowed them to stay human, sort of. The sex of the victim might be changed falling into a pool. Records were kept and over time it was determined there were at least four pools. The pools were voluptuous teenage girl, muscular teenage boy, virtuous man and priest. A man falling into the boy pool would just be altered to his younger form until changed back. There was no change to the man's form when cursed by virtuous man or priest, he would however be virtuous or priestly in attitude until changed back. Unfortunately, a man cursed by pool of the girl would change into the form of what he would look as if born female.
Over time everyone accepted the tale of something drowning in a pool was what produced the curse. Locals have no idea why the pools exist and can only think that Kami-sama has a reason.