At least, I think that's what it was called.

I don't really feel like doing the research on it, seeing how I don't play the game anymore, but... story time!

So I was playing my Leaf Green, and I'd just beaten the Elite Four for the second time and I thought I'd go around and catch some pogeys. I wanted a Milotic, so I caught a Feebas. When I got to the Pokemon Center, and my pogey's were healed, Nurse Joy said something about one of my pogeys having a Pokerus. I checked the status of the Feebas, and sure enough it had "Pokerus" labeled to it.

Intrigued, I carried it around for a bit, checking up on my pogey's every so often. I suddenly noticed that another one of my pogeys also had the Pokerus. Alarmed, I went back to the Center (by then a third pogey was infected) and put the three of them in a separate box named "Pokerus."

That was a few months ago... I haven't gone back to it since. Actually, that's a lie; I went once, like last week, and sure enough the Pokerus guys are still infected.

So... what IS this thing?