Bubonic plague
Alexander was a Hungarian warlord who lived in the early 5th century. He ascended to power just in time to watch his village ravaged by an unknown plague. Alexander was the only survivor of the plague. His body was able to adapt the virus, and, likewise be adapted by it, and through some unknown means consequently made him immortal. The plague that wiped-out his home town and mutated with Alexander "may" have been a part of the first wave of the Bubonic plague to hit Europe (otherwise known as the "Plague of Justinian"), although the timing for the Justinian Plague is off by a nigh-century (although archaeo-pathological evidence reveals that Bubonic plague had been around as early as the Bronze Age), and that Bubonic plague is bacterial rather than viral.