Chronic Endemicity
    Birds are the hosts for West Nile virus.  While the exact species are not fully known, the corvids (crows) are suspected to be a primary host.  Mosquitoes carry the virus, they bite humans and birds, and the virus is spread in this way.  When infected, a bird either dies or becomes immune.  As time passes the susceptible adult birds all die off, and in a new season the only birds that can become infected and spread the virus are the baby birds.  That is the point of chronic endemicity, and with the greatly reduced infected bird population human infections have by then come way down.