If the NME salesguy did send over the virus, why would he infect himself? (Response: Maybe the NME sales guy was faking it.)
Apparently, Waddle Dees, Waddle Doo, and Kirby are immune to colds (Salesguy seemed more interested in making money than eliminating Kirby, and only mentioned having Kirby suck up the monsters to sell the Microtizer). Then again, if Kirby's immne, how did Meta Knight get sick? (the games at least reveal that Kirby and Meta Knight are of the same species)
Wearing masks when you're sick is custom in Japan. Amazingly the dub didn't edit those scenes or skip the episode entirely.
It's never revealed how everyone else got sick. If only Kirby could defeat the virus monsters, then they either die naturally over time, or Dream Land just happened to be hit with a cold epidemic (it's also likely they DID send the virus, but how many are within the victim's body determine's the duration of the cold).