Despite the attention being paid to avian influenza and its potential to cause a pandemic among humans, the threat of avian diseases extends well beyond the influenza virus. Of particular interest to poultry producers are foodborne infectious bacteria such as Salmonella and Campylobacter spp., as well as several viruses, including those responsible for causing Exotic Newcastle Disease, avian leukosis, and Marek's disease (MD). Indeed, Marek's disease virus (MDV), an oncogenic avian herpesvirus that causes the lymphoproliferative disease characteristic of MD, is becoming more virulent and evading control by available vaccines—thus posing an increasing challenge for poultry producers.