Star Trek: 10 Things You Didn't Know About Section 31
3. Section 31 Giving Odo The Morphogenic Virus Was Writer Rene Echevarria’s Idea
During the Dominion War, Section 31 developed the morphogenic virus, a wasting disease that attacked a Changeling's morphogenic matrix, the structure that gave them their shape-shifting abilities. The disease progressed as individual Changelings changed form. The more rapidly they changed shape, the faster the disease progressed. This side-effect had Section 31's desired effect of hampering the Changelings' infiltration efforts.
Section 31 had infected Odo during a medical exam when he and Sisko were on Earth. Odo spread the disease when he linked with other Changelings. Although originally intended as a carrier similar to Typhoid Mary, Odo eventually developed the disease.
Writer Rene Echevarria revealed that Section 31 infected Odo in the episode When It Rains. He felt that the revelation that Section 31 was responsible for creating the virus didn’t raise the dramatic stakes enough. They're a clandestine intelligence agency, creating biological weapons is just what they do.
This reveal gave the writers a way to end the Dominion War. In the episode Extreme Measures, Bashir and Miles O'Brien extracted the cure from Luther Sloan. In What You Leave Behind, the Female Changeling ordered her forces to surrender after she received the cure from Odo.