What was so special about Corvinus that he and his descendents were immortal?
Alexander Corvinus is a Hungarian warlord who ascended to power in the early fifth century, just in time to watch his village ravaged by an unknown plague. Alexander was the only survivor of the plague. His body adapted to the virus, and through some unknown means consequently made him the first true immortal, by the Immortal gene.
and form Immortal gene underworld.wikia
A plague wiped out his entire village, but Alexander had a very rare blood type that allowed him to develop immunity to the virus and instead of killing him, prolonged his life. This ability was passed down to two of his three sons.
The third Corvinus son had this gene passed down through the generations until it was found lying dormant in Michael Corvin.