The rapid growth of the global human population has led to an urgent need for affordable safe food. Since poultry meat can help satisfy this need the global poultry industry has experienced significant growth in recent years. Pathogenic avian viruses represent a major threat to the industry. Although a large number of viruses infect poultry, some avian viruses cause in apparent infections while others cause severe diseases with economically devastating consequences. In addition some avian viruses are zoonotic thus capable of infecting humans. For example, the highly pathogenic avian influenza virus subtype H5N1 can infect humans, often having fatal outcomes.