An airborne disorder that activated dormant genes and turned the Enterprise-D crew into earlier evolutionary forms, named after Reginald Barclay, its first patient.
Barclay's Protomorphosis Syndrome is the affliction in the Next Generation episode Genesis. Beverly Crusher treats Barclay's mild Urodelan flu by activating a dormant T-cell with a synthetic one; an anomaly in his genetic make-up causes the cell to switch on every dormant intron in his body and then to spread through the ship.
The crew regress along their species' evolutionary paths. Riker becomes a primitive hominid, Troi an amphibian, Barclay a spider-like creature and Worf a venomous armoured predator, while the ship drifts.
Picard and Data return from a shuttle mission to find the disaster. Data builds a retrovirus from the amniotic fluid of his pregnant cat Spot, and Crusher names the syndrome after the lieutenant, who is quietly pleased.