Avian Pox has been prevalent in species such as House Sparrow and Woodpigeon for a number of years. Despite some claims to the contra with House Sparrows, avian Pox cannot be blamed for any reduction in the Woodpigeon population; at least, not in Northrepps, Norfolk.
The virus has now undergone a mutation and has entered the Great Tit Population. From what I read it is not going to kill off the species but make it vulnerable to disease. It apparently manifests itself in the Great Tit population in the form of lesions and sores. Not nice.
Keep those feeders disinfected.
Letter From Norfolk 