Convenient rain for dramatic purposes strikes again!
And any character in a manga that gets the slightest bit wet in a rain shower suffers from an instant and almost fatal cold!
There is one known cure: rice porridge, cool cloths on the forehead, and the most essential, taking off the victim's top and wiping the sweat off their body. You know it's working if their forehead feels less hot when you push yours against it. If they're still hot or sweaty and turn red, you need to keep them topless and alternate more body wiping and spoon feeding until they recover. Don't be afraid to fall asleep next to them. Also, if you catch their cold, that will cure it instantly.