The rakghouls were deformed mutants living in the Undercity of the Outer Rim world of Taris. They plagued the inhabitants of the Undercity- criminals or their descendants- with their bites having the power to mutate their victim into another rakghoul (which seems to happen rather drastically some time after infection, as Revan witnessed).
During the occupation and blockading of Taris by the Sith, the Sith troopers assigned to search for crashed escape pods in the Undercity were given a serum that could be used to counteract the effect of a rakghoul bite. However, Revan, by now suffering a memory wipe, managed to kill several Sith infantrymen in patrols, and stumbled across a Sith squad butchered by the rakghouls, and on one of their cadavers he found a small portion of the serum. This, he handed over to the Upper City doctor Zelka Forn, whom began extensive production of the cure, and distributed it throughout the Undercity. Unforunately, that may have been of little consequence whatsoever as the planet was destroyed by a Sith armada headed by Darth Malak a short while after.
It is possible that the rakghouls survived the bombardment, along with the outcasts of the Undercity.