John H. Beigel, MD, clarifies the definition of influenza and discusses the evolution of viruses, speculation on the mode of transmission and the role of vaccines and therapies as they relate to Influenza A (H1N1). The conversation references his recent publication in Critical Care Medicine (Beigel JH. Influenza. Crit Care Med. 2008; 36:2660-2666). Dr. Beigel is Director of Clinical Research at MacroGenics, Inc. in Rockville, Maryland, and a volunteer consultant at the National Institute of Allergy and Infectious Diseases at the National Institutes of Health. This podcast is the second in a series focused on the H1N1 influenza outbreak.