Dr. Marc Siegel is a practicing internist and professor of medicine at the NYU School of Medicine. He recently authored two books: "Bird Flu: Everything You Need To Know About The Next Pandemic" and "False Alarm: the Truth About the Epidemic of Fear" (Wiley).

H5N1 Virus: Avian Influenza

"Avian Influenza" is actually a group of illnesses caused by many different viruses, all of the same species: "Influenza A" (the "A", by the way, does NOT stand for "avian"). But one of its subspecies,"H5N1," has killed nearly two thirds of those in whom it has been diagnosed. And it apparently makes so many different animal species sick that some public health officials warn of a worldwide pandemic.

Producer: Dave Brody Editor: Chris Rodriguez