From the Centers for Disease Control and Prevention:
No symptoms in most people. Most people (70-80%) who become infected with West Nile virus do not develop any symptoms.
Febrile illness in some people. About 1 in 5 people who are infected will develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months.
Severe symptoms in a few people. Less than 1% of people who are infected will develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues).
The symptoms of neurologic illness can include headache, high fever, neck stiffness, disorientation, coma, tremors, seizures, or paralysis.
Serious illness can occur in people of any age. However, people over 60 years of age are at the greatest risk for severe disease. People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk for serious illness.
Recovery from severe disease may take several weeks or months. Some of the neurologic effects may be permanent.
About 10 percent of people who develop neurologic infection due to West Nile virus will die.
No vaccine or specific antiviral treatments for West Nile virus infection are available.
Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms