A minor flu to which most humanoids are naturally immune, which Doctor Crusher treated in Reginald Barclay with a synthetic T-cell that accidentally began the crew's de-evolution.
Urodelan flu is a trivial infection that most people fight off without noticing, thanks to a particular gene. Reginald Barclay's copy of that gene happens to be dormant, so when the nervous engineer reports to sickbay in the Next Generation episode Genesis, convinced he has the deadly Terellian death syndrome, the flu is all Doctor Crusher finds.
She activates the dormant gene with a synthetic T-cell, a routine treatment. Because of an anomaly in Barclay's genetic make-up the T-cell mutates, becomes airborne and starts switching on dormant introns in everyone aboard, the ancient genetic leftovers of each species' ancestors.
The result is the condition later named Barclay's protomorphosis syndrome. Picard and Data return from a shuttle mission to find the crew regressing into primitive forms, among them a spider-like Barclay, an amphibious Troi and a ferocious armoured Worf.