Phage
The Phage is a pandemic disease that affects most of the Vidiian race in Star Trek. Despite their advanced medical technology, which is substantially superior to that of the Federation, the Vidiians are unable to cure the Phage. The Phage is apparently not contagious to non-Vidiians, at least not through casual contact; Humans and other humanoids have spent prolonged periods in the company of Vidiians without being infected.
Symptoms
The Phage damages tissue throughout the body of the victim, leading to extreme disfigurement and frequent organ failures. Even superior Vidiian medical technology can only slow the decline of the victim's health, not stop it. To survive, many Vidiians attack other humanoids to steal healthy organs and tissues, which they then transplant to Vidiians suffering critical organ failures.