Its a virus, sometimes it will just kill the one YB in the nest but mostly both, you need to spray your loft out with disinfectant VirconS is the best. Once the babies get to 4 days old they will live through it and be fine in most cases so dont go mad and kill them all yourself. I know several people who are very good pigeon men who have suffered this so get to work cleaning and disinfecting all surfaces of your loft, walls, ceiling the whole lot and you will find your nasty little problem will be a thing of the past. Never known it to strike the same loft twice so fingers crossed you wont be the first that happens to. Let me know how you get on I'm sure what ive said will sort it for you. Mark.