Pokerus Info.
Okay, here's what I know about Pokerus:
It's a virus, but it's a good one (Think of what happened to John Travolta in "Phenomenon," but without the death thing at the end, lol). When your Pokemon has Pokerus, it will gain double the EVs (effort values) every time you battle. The effects only last for three days, however, so you have to be fast.
Pokerus can be held in a suspended animation of sorts by putting the affected Pokemon in the PC. It won't lose it until it's spent three days outside. So the smartest thing to do if you ever catch it is to infect a bunch of pokemon and put some of them in your PC for safekeeping. That way you can infect others later on.
Oh, and I forgot. You can spread the virus to your other teammates. If the "afflicted" pokemon is in your team, and you battle, there's a chance that the pokemon on either side of him/her will catch Pokerus, whether they or he battles or not. So ideally, you can inflict Pokerus on the Pokemon you want to EV train, and they'll train faster.
EV training is actually very good, especially if you ever want to battle your pokemon against live opponents like your friends. Having a super-fast Pokemon, or one with Super-attack can be really helpful in certain situations. I recommend it if you're a real hardcore trainer, but it can make progressing through the game a real pain.
You can tell if your pokemon has pokerus by its status area (the little area below its picture on the stats page). Although I don't know if it appears there before or after you're diagnosed at the Poke center. Oh yeah, and Nurse Joy will let you know if one of your Pokemon has Pokerus, but only once. She won't tell you again after that.
And finally, for those who want to catch pokerus, you can do one of two things. A: you catch a pokemon that has it. B: you battle a wild pokemon that has it, and it infects you. I'm pretty sure that you only have a certain percentage chance of catching it via B, but I may be wrong. Anyway, it's so hard to get that I wouldn't recommend catching every Pokemon you find to try to get it. It's like the Shinies. Just go about your business and forget about it. It may show up when you least expect it!