A mutated cold virus was featured prominently in the episode "Bound".
The Mutate Cold Virus is what Peter, Walter, and Olivia describe as,
Walter: "This organism is a single specimen of a virus for the common cold."
Olivia: "So you're saying that this is one single cell?"
Walter: "It's not unprecedented. The ostrich egg is a single cell, and it can grow up to five pounds."
Peter: "They supersized the common cold, which, as it turns out, is disgusting."
Olivia: "Obviously somebody's playing with us."
Peter: "Yeah, they're showing off. Killing an epidemiologist with the common cold."