Children of the Night — Dan Simmons

In a desolate orphanage in what remains of post-Communist Romania, a desperately ill infant is given the wrong blood transfusion--and flourishes when he's sup-posed to die. The discovery of his unique immune system may hold the key to the long-awaited cure for cancer and AIDS. For a dedicated American doctor, he promises the medical breakthrough of a lifetime, as well as a very special love she's never been able to find. But he also conceals a shockingly intimate link to a clan of vampires and their legendary leader--the fiend the world calls Vlad Dracula, who, for centuries, has triumphed over countless rival tyrants, including death itself...