Many viruses definitively cause disease in our companion birds, whereas other viruses have been implicated or associated with typical clinical signs. Some families of viruses that have been discovered in mammals have not been associated with disease in birds. It is imperative to perform a necropsy on any birds that die--whether a pet, aviary, or display bird, and despite the fact that other diseases may be present--because viruses can occur concurrently, especially when immunosuppression is present. Also, it is imperative to use available vaccines to decrease and control the incidence of these diseases, as has occurred in the canine and feline pet populations.