The phage was a deadly disease that afflicted the Vidiians for approximately 2,000 years which killed thousands daily. The disease was highly adaptive to medical treatment. It consumed the Vidiians, at first disrupting their genetic code, then consuming their body tissues and destroying cellular structures. 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 euthanasia of afflicted persons was common. (VOY: "Phage")
The Vidiians survived in the short term by making extensive use of transplantation and organ piracy, using 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. (VOY: "Faces")
The phage was cured by 2375, by the group known as the Think Tank. (VOY: "Think Tank")