Jenny,
That's what is so weird about this critter. It can come on like a freight train, rot off half of your fishe's faces, then disappear. The good news is it's being researched pretty heavily.
To tell you the truth though, I wouldn't panic too much about it. I would seriously doubt that you have the same deal in your tank. I would, though, do a 50% water change, raise your temp to something around 81 degrees, add about 1/4 cup salt per 10 gallons (providing you're not dealing with salt-sensitive fish), and add a decent broad-spectrum anti-bacterial/anti-parasitic medication. Just to be safe, I would move your apple snail to another container while you're treating your water.
All of this is suggested under the assumption that your water is in good shape, you're not using an under gravel filter that is so clogged it's anerobic, and there isn't anything in there that would try to eat your fish. Aside from that, it should clear up pretty quickly.