There are about seven principal mosquito-borne viruses that are competent of crossing the blood-brain barrier in humans and other animals in the United States, causing an acute infection of the central nervous system. These include Western Equine Encephalitis (WEE) and St. Louis Encephalitis (SLE) which have been known to occur in the state of Colorado.
For example, the West Nile Virus, in 2003, Colorado recorded the first large scale human epidemic of mosquito borne disease on record. Although the exact reasons for this wide-spread epidemic are not entirely clear, Colorado's wet spring and hot summer certainly played a critical role. These conditions created an abnormally large and much earlier than normal hatch of Culex tarsalis mosquitoes. This hatch was approximately one month earlier than normal based on Colorado Mosquito Control trapping records over the previous nine years on the northern Front Range. We feel that this early and large hatch of Culex mosquitoes allowed the virus to replicate and spread rapidly through the bird and existing adult mosquito populations, which in turn infected the rapidly increasing Culex populations and eventually allowed the virus to spread to other animals and humans.