A disease created by Section 31 to wipe out the Founders of the Dominion, secretly given to Odo so that he would carry it into the Great Link.
The morphogenic virus is a slow-acting illness that attacks Changelings, causing their bodies to dry, flake and finally lose all cohesion. Section 31 devised it as a means of genocide and infected Odo without his knowledge during a visit to Starfleet Medical, and he passed it on when he later joined the Great Link.
The plague comes to light in the last season of Deep Space Nine. The Female Changeling is visibly ailing, and in When It Rains... Bashir finds that Odo is infected and traces the disease to Section 31. In Extreme Measures he and O'Brien lure Sloan to the station and enter his dying mind to extract the cure.
Bashir heals Odo, and in What You Leave Behind Odo links with the Female Changeling and cures her, persuading her to end the war. He then returns to the Great Link to heal his people.