Yeah the disease is called Trichomonosis and was the reason Greenfinches (in particular) took such a blow to the population around 2006/7. Think it was the largest scale mortality of birds due to disease on record (in the UK), and it's continuing to this day to reduce their numbers sadly. They believe woodpigeons, doves etc are prone to the disease, but the main way it infects other birds is through dirty feeders and water containers/baths so it is really important to keep everything clean. Also a good idea is to have multiple feeders (or rotate existing ones) to prevent the build up of contamination in any one area, and to stop a lot of birds gathering on one feeder.
We had one stuck in our feeder in November, friendly little thing and was hardly scared of us - couldn't believe it! We also had a family of goldfinches and chaffinches, but they've all disappeared recently. Have still got the loyal robins and other wee birds though, and some blackbirds. Love them all but wish our goldfinches would come back lol
If we get this cold spell at least there's plenty on here looking out for the wee birds