Encountered by USS Voyager in 2371, the Vidiians suffer from a disease known as "the Phage" which attacks every part of their bodies. The Vidiians have no cure for the Phage, and are forced to replace body parts on a regular basis. Despite possessing advanced medical knowledge, including genetic engineering, the Vidiians apparently lack the ability to clone bodies for replacement parts. They therefore frequently attack other species and steal their organs for transplants.
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This has led to the average Vidiian having a wide variety of alien organs and skin on their body, resulting in a hideous 'Frankenstein' appearance. Voyager first encountered the species when two Vidiians stole a lung of Neelix whilst he was on an away mission,  and have subsequently attacked the ship or crew on several occasions.  Vidiian technology is generally in advance of that of the Federation, making them formidable opponents. 
Voyager passed out of Vidiian space sometime after late 2372 but although there was no further direct contact, this was not the last the Federation ship has heard of the species. In 2375 Voyager was contacted by the "Think Tank", a small group of aliens who specialize in solving problems for other species. The Think Tanks representative, Kurros, informed Captain Janeway that his group had recently found a cure for the Vidiian Phage.  It can only be hoped that with the end of this great burden on their civilization, the Vidiians returned to a more peaceful lifestyle.