The vast majority of bird species appear to be susceptible to infection with APMV-1 of both high and low
virulence for chickens, although the clinical signs seen in infected birds vary widely and are dependent on
factors such as: the virus, host species, age of host, infection with other organisms, environmental stress
and immune status. In some circumstances infection with the extremely virulent viruses may result in sudden
high mortality with comparatively few clinical signs. Thus, the clinical signs are variable and influenced by
other factors so that none can be regarded as pathognomonic.