House Finch Eye Disease: Increased virulence as disease progresses

Usually, individual birds infected with a disease develop an “immune memory” that is used to help protect them from reinfection. It’s similar to when your body creates antibodies after you get sick or receive a vaccine. The antibodies help prevent reinfection by that same pathogen strain. However, since birds can’t walk into vaccine clinics, their immune memories are not always perfect, and we’re learning about this phenomenon from House Finch eye disease.

https://feederwatch.org/blog/house-finch-eye-disease-increased-virulence-disease-progresses/