A mutated cold virus was featured prominently in the episode "Bound".
The normal common cold is a viral infectious disease of the upper respiratory system. Caused by common viral strains of rhinoviruses and coronaviruses, it is rarely fatal by itself, although unable to be vaccinated against due to many mutational strains. Z.F.T engineered an unknown strain of the disease become "supersized," manifesting as a foot-long, slug-like, single-celled creature capable of locomotion.
The mutated virus is stored in an inert yellow powder form which can be dissolved into water; once ingested and mixing with stomach acids, the virus rapidly mutates into its slug form. From here, it causes suffocation and internal damage as it forces its way up the esophagus and out the victim's mouth, killing them.
Mitchell Loeb, working as a ZFT operative within the FBI, oversaw a warehouse where the virus was stored; however, when he abducted Oliva Dunham and imprisoned her at said warehouse, she escaped and stole samples of the viral weapon. Walter quickly determined the nature of the samples, which had been used to kill immunologist Miles Kinberg and epidemiologist Russel Simon by Loeb.
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."