During the years following the discovery of West Nile Virus in Illinois, Crow populations dropped from 300,000 to 110,000 birds. According to INHS Ornithologist Mike Ward, crows were highly susceptible to West Nile Virus, possibly because of their specialized immune systems and social lifestyle.  The cause of the rebound is not fully understood, but Ward stated that antibodies to fight off the virus have been discovered recently. Wet conditions during the summer may have also decresed breeding of the mosquito species that carries West Nile Virus.