Pokérus, the epic boost
When I first got Pokérus, I too thought it was bad. However, your friend is correct in this case. Pokérus is an enhancing virus. In other words, the disease is actually a good thing! :D It's rare and started way back in Gold & Silver. When a Pokémon's status reads Pokérus, you can possibly infect your other party members with walking around, battling, etc. :D
"After a Pokémon's immune system fights off the virus, it will still be able to gain double Effort Values permanently." (Which means, you get a huge boost for EV training) Aka stats grow twice as fast. :D
Once a Pokémon is cured, they get a little smiley face, that shows they have successfully handled Pokérus. :) Once it's cured, it can't infect any other party members, so hopefully infect while you have it.