I paused before posting this image because the scene is horrible, but the strain of Avian Pox virus that has afflicted this Great Tit is a fairly recent arrival in the UK and tracking its progress and prevalence must be worthwhile. This species is particularly susceptible, and bird feeders that will bring many birds to feed at the same location, may well be a source of cross-infection. Feeders and bird baths can be safely sterlised with a variety of bird-safe treatments (or boiling water if they are not plastic).
This bird was still actively feeding, but it was blind in one eye and seemed very unsteady. A truly revolting disease.