West Nile Virus:
How Mayo Clinic Defines It:
West Nile virus is an infection transmitted by mosquitoes. If you become infected with West Nile virus, you may not experience any signs or symptoms or you may experience only minor ones, such as fever and mild headache. However, some people who become infected with West Nile virus develop a life-threatening illness that includes inflammation of the brain.
Mild signs and symptoms of a West Nile virus infection generally go away on their own. But severe signs and symptoms — such as a severe headache, disorientation or sudden weakness — require immediate attention.