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. The only non-Vidiian to be infected, a fully Klingon version of Belanna Torres created by Vidiian technology, was deliberately infected by a Vidiian doctor.
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.
Treatment
To survive, many Vidiians attack other humanoids to steal healthy organs and tissues, which they then transplant to Phage victims suffering critical organ failures.
The Klingon "clone" of Belanna Torres quickly recovered from the Phage after being exposed to it. Vidiian doctors are attempting to use samples of Klingon tissue to develop a cure.