The virus made a Changeling's morphogenic matrix unstable, leading to them becoming desiccated and unable to hold humanoid forms. (DS9 episode: "Chimera") A Changeling's use of their abilities also hastened the disease. (DS9 episode: "Tacking Into the Wind") When the virus took hold, it left the Changeling unable to revert to their natural form or use their abilities at all. (DS9 episode: "What You Leave Behind")