Spread by mosquito bite, found in birds.
Meningitis, encephalitis, can mimic polio, most are asymptomatic. Patients who have a deletion in the CCR5 chemokine are much more likely to get encephalitis (PubMed), but less able to get HIV. Win one, lose one. As of 2010, the CDC is reporting the that incidence for neuroinvasive disease has decreased five fold: 1.02 to 0.23 per 100,000 population between 2002 and 2008. Is the organism mutating?
A small subset of patients can be ill for years and have virus isolated from urine (PubMed).
ICD9 Codes (Soon to be supplanted by ICD10)
West Nile encephalitis 066.3, West Nile fever 066.3.