Plasmavores were identical in appearance to humans. By draining the blood of an individual, the Plasmavores could mimic the internal structure and genetic material of their victim. More extensive mimicry would require more feeding.
The Plasmavores were created by the Xeraphin to wipe out the Vardon who captured the Xeraphin gestalt. (PROSE: The Vardon Horse) They became known as a dangerous species, travelling the universe and looking for new species to feed on.
One Plasmavore, known as Florence Finnegan, fed on the Child Princess of Padrivole Regency 9 on a whim and was then hunted down by the Judoon. She escaped to Earth and hid in the Royal Hope Hospital, feeding on the hospital's blood storage. She was protected by two Slabs. When the Judoon took the hospital to the Moon, she fed on Dr B. Stoker so that she could fool the Judoon scanners. She planned to kill everyone with an overpowered MRI machine and then escape on the Judoon's ship, but she also fed on the Tenth Doctor, giving her alien genetic material. This allowed the Judoon to recognise her as an alien and disintegrate her. (TV: Smith and Jones)