Vidiian

The Vidiians are a humanoid spacefaring species native to the Delta Quadrant of the Milky Way Galaxy. As of the 24th century, their civilization has suffered from an incurable disease known as the phage, an epidemic which has endured for over two thousand years and completely altered the evolution of their society, making them almost exclusively dedicate themselves to medical research and interspecies organ harvesting. Their government is known as the Vidiian Sodality.

Biology and society
Because the phage results in the degeneration and destruction of body tissues, the once prosperous Vidiians were soon converted in a society of pirates devoted to abducting members of other races for organ harvesting. Most Vidiians have a hideous appearance, their bodies consisting of a humanoid amalgamation of alien parts at different stages of degeneration.

Curiously, they not always kill their victims directly and will sometimes harvest a single organ (for example when they attacked Neelix and removed only his lungs, leaving the rest of him intact). No definite explanation has been given for this apparent waste which seems strange for a society so desperately needing transplants.

The phage doesn't seem to be hereditary. It appears that Vidiians are born healthy and there are individuals in existence which haven't been affected by the phage. Despite this, all efforts towards isolation and quarantine have been useless and the phage continues to affect the vast majority of their people.

In the year 2375 (three years after the last known encounter with the Vidiians) the multi-species intellectual society known as the Think Tank claimed that they had cured the Vidiians from the phage.

Technology
As a result of centuries of directing efforts towards fighting the phage and perfecting surgical techniques, their medical science is far more advanced than that of the Federation. They often carry a hand device which combines the functions of scanner, surgical instrument and weapon.

They are also masters of cloning and genetic engineering, as shown by their ability to split B'elanna Torres into two individuals, one fully Klingon and one fully Human. The fact that even this level of medical abilities isn't enough to free them the need to steal alien organs goes to show the extent to which the phage affects them. It is also possible that they judge organ cloning too expensive or ineffective, and that it is simply more practical to steal fresh organs.