Pokerus
When battling, there is a 2 in 65,535 chance that your Pokemon will become infected with the Pokerus virus; thus, most players will never see this virus in their game. However, if you do, you should feel very lucky. If you don't believe that this virus exists, look in Nintendo Power Volume 144 in the Pokemon Center to see Nintendo admit that this virus exists.
If your Pokemon has the virus and you look at its status, it will read, "POK�RUS". If you go and talk to Nurse Joy or Professor Elm they will explain that the virus is harmless and will wear off over time. The virus is also very contagious and can easily infect your whole party or even one of your PC boxes. If you're interested on how long it takes to go away, it takes about 5 days for a Pokemon's immune system to fight off the virus and when the Pokemon's cured, it's cured forever! If a Pokemon is cured forever a red dot will appear on their status screen.
Pokerus may sound like a bad thing, but it's actually good. If a Pokemon is infected with the virus, its stats grow twice as fast as normal, while still receiving normal experience for level. So if your Pokemon catch Pokerus, don't be scared, be happy because it will help your team, not hurt it.