West Nile virus is mainly transmitted to people through the bite of an infected mosquito.

Mosquitoes transmit the virus after becoming infected by feeding on the blood of birds which carry the virus.

Some individuals have weaker immune systems, and they are at greater risk of developing symptoms and health effects that are more serious, including meningitis, encephalitis and acute flaccid paralysis. Meningitis is inflammation of the lining of the brain or spinal cord. Encephalitis is inflammation of the brain itself. Acute flaccid paralysis is a polio-like syndrome that can result in the loss of function of one or more limbs. These conditions can be fatal.

For people with more severe illness, symptoms could include the rapid onset of severe headache, high fever, stiff neck, nausea, difficulty swallowing, vomiting, drowsiness, confusion, loss of consciousness, lack of coordination, muscle weakness and paralysis. Other symptoms that have been identified include movement disorders, parkinsonism, poliomyelitis-like syndrome and muscle degeneration. Anyone who has a sudden onset of these symptoms should seek immediate medical attention.

Who is at risk for serious health effects from West Nile virus?

While persons of any age or health status can be at risk of developing serious health effects from West Nile virus, the overall risk of serious health effects increases with age. People with weaker immune systems are considered to be at greater risk for serious health effects.

People at higher risk for serious health effects from West Nile virus include:
people with chronic diseases, such as cancer, diabetes, alcoholism, or heart disease;
people that require medical treatment that may weaken the immune system, i.e. chemotherapy.

Because West Nile virus can cause severe complications for people of any age and any health status, it is extremely important to reduce the risk of getting bitten by mosquitoes. Anyone exposed to mosquitoes in an area where West Nile virus has been detected is at some degree of risk for infection.