Children of the Night by Dan Simmons

While working on a rare blood disease problem in Romania, hematologist Kate Neuman decides to adopt a seven month old boy that requires biweekly blood transfusions to survive. She discovers that baby Joshua's unique immune system holds the keys to a cure for cancer and AIDS, but just when research promises a breakthrough the boy is taken, leading Kate to Transylvania, where she must face the historical Vlad Dracula.

Paperback Info: ISBN 0446364754, Warner Books, 1992
Hardcover Info: ISBN 0399137173, Putnam