Barclay's Protomorphosis Syndrome was accidentally created when Doctor Beverly Crusher injected Lieutenant Reginald Barclay with a synthetic T-cell to help cure his mild case of Urodelan flu. As designed, the T-cell was supposed to activate the genes that would give Barclay an immunity to the disease, but the T-cell accidentally activated all of his genes, including his introns - or genetic sequences that were no longer used but held in a person's DNA. The T-cell interacted with the Urodelan flu virus, and rapidly became airborne. This new virus infected everyone else on board the Enterprise, and activated their introns as well.
The crew of the Enterprise began showing symptoms in the hours after the virus became airborne. A number of crew members began exhibiting strange behavior, such as increased aggression, changes in activity level, or finding the environment uncomfortable. Soon after the crew began de-evolving into earlier forms of life.
Because Lieutenant Commander Data was an artificial being, he was not affected by the disease. As Captain Picard was off the ship when the virus initially spread through the ship, he was not immediately affected either. Arriving back on the Enterprise, the two found the crew had all de-evolved into earlier forms of life. Data and Picard found that Spot had recently given birth to a litter of kittens. While Spot had been transformed into a lizard, none of the kittens had been affected by the T-cell virus. This was due to the protection offered to the kittens by Spot's amniotic fluid. As Nurse Alyssa Ogawa had become pregnant herself recently, Data was able to use some of her amniotic fluid to construct a retrovirus, curing the crew and restoring them to normal.
Once cured, Doctor Crusher conducted research into recent events and discovered the origins of the disease. Informing Barclay of her findings, she offered to name the disease after him. After Barclay agreed, Crusher named the disease Barclay's Protomorphosis Syndrome (also called Barclay Syndrome). Upon learning that in addition to being transformed into a spider he had a disease now named after him, Deanna Troi remarked that she had better clear her calendar for the next few weeks. (TNG episode: "Genesis", ST website: Startrek.com)