Final answer:
West Nile virus is transmitted through Culex mosquitoes and affects mostly birds, with humans as incidental hosts. While most infections are asymptomatic, severe cases can lead to encephalitis or meningitis with a 10% mortality rate in such instances.
Step-by-step explanation:
Characteristics of West Nile Virus
West Nile virus (WNV) is primarily transmitted to humans through the bite of an infected Culex mosquito, which acts as a vector for the disease. Mosquitoes acquire the virus from feeding on infected birds, which are the natural reservoirs of WNV. A characteristic feature of this virus is that 70-80% of infected individuals do not exhibit any symptoms. When symptoms do arise, they are typically mild and flu-like; however, in less than 1% of cases, individuals may develop severe neurological conditions such as encephalitis or meningitis, which can be fatal. The mortality rate for those with severe neurological disease is approximately 10%. The rapid spread of WNV in North America, starting from its introduction in 1999, can be partly attributed to migratory birds and changed mosquito feeding behaviors.