I got it from a Golem I obtained in a trade from my neighbor boy waaaay back in the days of Silver/Gold. I remember when I transefered the Golem into Pokemon Stadium 2, it was all red, so I thought that Pokerus was short for Pokemon Rust (see? You're not the only one, user who said that before me).
Shortly after, my Gold version corrupted. I thought the 'rus was to blame. I didn't have the internet, so I couldn't just look it up, and no one I knew had ever heard of Pokerus, so I didn't know any better.
Though I know it's good, now, I'm still a little leery of it. (I just got it on a Japanese Totodile I got off the GTS, as well as on a Turtwig from The Netherlands. I've got them quarintined, because, as previously stated, I'm still a little leery of it. Plus, my team's already fully trained, so what's the point?)
~Nemmeh