The Vidiian species are a violent race who are infected with a terrible, incurable illness: the Phage. To help dying comrades, they kill members of other races, and steal their healthy organs. Voyager has encountered them many times.
On one occasion, Neelix's lungs were removed and grafted into a Vidiian body, making them unretrievable. However, using their sophisticated medical technology, a Vidiian surgeon transplanted a lung from Kes into Neelix's body. During another encounter, B'Elanna was split into two people; one entirely Klingon and one entirely human.
And on a third occasion, a Vidiian troopship overwhelmed Voyager, killing most of the crew. When they reached the bridge, however, the alternate universe Captain Janeway had a surprise up her sleeve: she self-destructed her ship, taking the Vidiian craft with it.
