Morphogenic virus

The morphogenic virus was a disease created by Section 31 that was designed to wipe out the Changeling race and deprive the Dominion of its leaders.

Symptoms
The virus made a Changeling's morphogenic matrix unstable, leading to them becoming desiccated and unable to hold humanoid forms. 

A Changelings use of their abilities also hastened the disease. 

When the virus took hold, it left the Changeling unable to revert to their natural form or use their abilities at all. 

Infection
In 2372, while on Earth helping Starfleet to find ways to detect Changelings, Odo was infected with the disease by Section 31, who hoped he would link with other members of his race and infect them all. 

It was believed that Odo would not develop the virus and act only as a carrier. 

Taking effect
In 2374, Tomas Roeder discovered the existence of the virus. He attempted to smuggle classified data on Changeling morphology, the damage the virus was doing to it and the names of the designers and others involved to the Dominion. 

The Founders began suffering from the effects of the disease in 2375, with the entire Great Link being infected. The Female Changeling was also infected with the disease and her symptoms were noticed by Weyoun and Corat Damar. The Female Changeling informed Weyoun of the disease and the fifth clone of Weyoun informed Odo of it when he attempted to defect to the Federation. 

The Vorta began research into a cure for the disease, but had no success. The Female Changeling had them document their findings before ordering Weyoun to execute them and activate their clones, hoping for a "fresh perspective". 

Late in 2375, Julian Bashir discovered Odo had become infected with the disease and began research into a cure. Despite his efforts, Bashir made little headway and he and Miles O'Brien sent a message to Starfleet Medical that stated that Bashir had found a cure. It was hoped Sloan or another 31 agent would come to Deep Space 9 to confirm it, giving Bashir and O'Brien an opportunity to find a cure from the agent or give them the name of someone who would have it. 

The cure
Towards the end of the war, after successfully luring Luther Sloan to Deep Space 9, Bashir and O'Brien used a multitronic device to link their minds to Sloan's. They managed to find the cure before Sloan died after having committed suicide. 

The Federation Council debated giving the Founders the cure, although they decided against it, as they felt it could strengthen the Dominion's hand and allow them to win the war. 

The Female Changeling had become terminally ill with the disease and had been unable to regenerate. Following the Battle of Cardassia, Odo linked with the Founder and cured her of the disease. In return for having the Female Changeling order her forces to stand down, Odo agreed to return home and cure the Great Link as well. Odo returned to the Gamma Quadrant several days later and cured the Link. 

Legacy
The Female Changeling learned that the virus had been created by Section 31, probably during her link with Odo. Odo himself was concerned how the Great Link would react to the information when he returned home. (SS DS9 (Requital))

As Odo predicted, the disease and its creators cause discord in the Great Link, with some not accepting Odo's arguments that it was an extreme minority. 

Laas was infected with the disease after linking with Odo, but was cured by the Great Link when he returned home.