The phage is a deadly disease that afflicted the Vidiians for approximately 2,000 years which kills thousands each day. The phage is highly adaptive to medical treatment. It consumed the Vidiians; their genetic code was first disrupted, then their body tissues were consumed and cellular structures destroyed. One of the early symptoms of the disease was excruciating joint pain. Some victims had been known to die simply from the agony itself, and euthenasia of afflicted persons was common. (VGR: "Phage")
The Vidiians survived in the short term by making extensive use of transplantation and organ piracy, using a variant of transporter technology to literally steal healthy organs from the bodies of their owners. Increasingly desperate, the Vidiians resorted to kidnapping members of other species and experimenting on them.
A Vidiian scientist, Sulan, believed that research into Klingon genetic material would eventually lead to a cure. (VGR: "Faces")
The phage was cured by 2375, by the group known as the Think Tank. (VOY: "Think Tank")