Disease Carrier

I've observed several cowbirds recently with obvious outward manifestations of disease. Here the bird is obviously dirty from its overnight roost. One can also see an open sore on its foot, and a cyst of some type near the beak/eye. So I looked into brown headed cowbird diseases, and how they might possibly relate to or affect man.
They are known for being very resistant to disease, and are major carriers of West Nile virus and Sarcocystis Neurona (a disease mostly affecting horses). Hard to determine but it appears that sometimes these diseases are fatal for the birds. It turns out they are mostly just carriers though, with only a very small percentage of the birds actually dying..