Influenza A is caused by specified viruses that are members of the family Orthomyxoviridae and
placed in the genus influenzavirus A. There are three influenza genera – A, B and C; only influenza
A viruses are known to infect birds. Diagnosis is by isolation of the virus or by detection and
characterisation of fragments of its genome. This is because infections in birds can give rise to a
wide variety of clinical signs that may vary according to the host, strain of virus, the host’s immune
status, presence of any secondary exacerbating organisms and environmental conditions.