West Nile virus
Corvids are reservoirs (carriers) for the West Nile virus in the United States. They are infected by mosquitoes (the vectors), primarily of the "Culex" species. Crows and ravens are quickly killed by this disease, so their deaths are an early-warning system when West Nile virus arrives in an area (as are horse and other bird species deaths). One of the first signs that West Nile virus first arrived in the US in 1999 was the death of crows in New York.