Ditto. But if it had to happen to any fish in a community tank, betta is the best one for it to happen to, if you don't have a hospital tank.
I say take him out and put him in anything big enough to hold him and give him room to move around (something preferably not tall like a vase, even a small tupperware bowl will do). If you don't have an extra heater, put the bowl somewhere warm (on top of the tv, if it's always on like it is in my house
or maybe on a windowsill in the sunlight).
Go to LFS and get some Melafix - if that's not available look for something that will cure a fungus or infection. Treat the betta separately. But also do a water change in the tank asap, and keep an eye out for signs of the same thing on the other fish. If it doesn't pop up soon on other fish then they should be in the clear.
Good luck!
~Tara