West Nile virus is an arthropod-borne virus (arbovirus) most commonly spread by infected mosquitoes.

West Nile virus is most commonly transmitted to humans by mosquitoes. Additional routes of human infection have also been documented. It is important to note that these methods of transmission represent a very small proportion of cases:

• Blood transfusions
• Organ transplants
• Exposure in a laboratory setting
• From mother to baby during pregnancy, delivery, or breastfeeding 

West Nile virus is not transmitted:

• From person-to-person or from animal-to-person through casual contact. Normal veterinary infection control precautions should be followed when caring for a horse suspected to have this or any viral infection.
• From handling live or dead infected birds. You should avoid bare-handed contact when handling any dead animal. If you are disposing of a dead bird, use gloves or double plastic bags to place the carcass in a garbage can.
• Through consuming infected birds or animals. In keeping with overall public health practice, and due to the risk of known food-borne pathogens, always follow procedures for fully cooking meat from either birds or mammals.

No symptoms in most people: Most people (70-80%) who become infected with West Nile virus do not develop any symptoms.

Febrile (fever) 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.

West Nile virus disease cases have been reported from all 48 lower states. The only states that have not reported cases are Alaska and Hawaii. Seasonal outbreaks often occur in local areas that can vary from year to year. The weather, numbers of birds that maintain the virus, numbers of mosquitoes that spread the virus, and human behavior are all factors that can influence when and where outbreaks occur.

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.

There are no medications to treat or vaccines to prevent West Nile virus infection. Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms.

People with milder symptoms typically recover on their own, although some symptoms may last for several weeks.

In more severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

The most effective way to avoid West Nile virus disease is to prevent mosquito bites:

• Use insect repellents when you go outdoors. Repellents containing DEET, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products provide longer-lasting protection.
• Wear long sleeves and pants from dusk through dawn when many mosquitoes are most active.
• Install or repair screens on windows and doors. If you have it, use your air conditioning.
• Help reduce the number of mosquitoes around your home. Empty standing water from containers such as flowerpots, gutters, buckets, pool covers, pet water dishes, discarded tires, and birdbaths.

West Nile virus disease is a nationally notifiable condition. Cases are reported to CDC by state and local health departments using standard case definitions.