Gerhard Adam Schad, 91, of Chadds Ford, a professor of parasitology at the University of Pennsylvania School of Veterinary Medicine for more than 35 years, died of cancer on Saturday, April 25, 2009 at his home. Dr. Schad, whose research was regularly funded by the NIH, did groundbreaking studies on the biology of human hookworms.    Dr. Schad was past president of the American Society of Parasitologists.