Paridae Pox is a virus infection which causes ulcerative, nodular, proliferative lesions, particularly on the head, neck and wings, and particularly in the Great Tit (Parus major). Some affected individuals recover, but often diseased birds will succumb to other causes of mortality because of the effect on flight. feeding, inability to escape from predators etc. While recovery is said to be possible, affected individuals I have observed in my garden have tended to doze while perched, gradually losing strength until they are no longer viable. In the UK the incidence is greatest in August and September, coinciding with the highest populations of biting insects, such as mosquitos, which are thought to be the principal vectors.