The West Nile virus is spread by mosquitoes. The West Nile virus can infect humans, birds, mosquitoes, horses, and some other mammals. In very rare cases, the virus can spread in transfused blood, a transplanted organ, or through the placenta to a fetus.

West Nile virus infections occur in mid- to late summer and early fall in mild zones. It can also occur year-round in warmer or southern climates. Most often, the West Nile virus causes no symptoms or a mild, flu-like illness. But the virus can cause life-threatening illnesses, such as:

• Inflammation of the brain (encephalitis).
• Inflammation of the lining of the brain and spinal cord (meningitis).
• Inflammation of the brain and its surrounding membrane (meningoencephalitis).

West Nile virus is spread to humans through the bite of an infected female mosquito. The mosquitoes get the virus when they bite an infected bird. Crows and jays are the most common birds linked to the virus. But at least 110 other bird species can also carry the virus.

West Nile virus isn't spread between humans. But there have been a few cases where it has spread through organ transplants. The risk of getting the virus from an organ is unknown. Not all organ donors are tested for West Nile virus. All blood is screened for the virus. The risk of getting West Nile virus from blood is much lower than the risk of not having any procedure that would call for a blood transfusion.

Most people infected with West Nile virus have no symptoms. They won't know they had the infection.

About 1 in 4 people who become infected will develop West Nile fever. This is a flu-like illness. The most common symptoms of West Nile fever include:

• Fever.
• Headache.
• Body aches.
• A skin rash on the trunk of body.
• Swollen lymph glands.
• Joint pains.
• Vomiting.
• Diarrhea.
• Fatigue.

The more severe form of the West Nile virus affects mostly older adults and those with weak immune systems. It occurs when the virus crosses the blood-brain barrier and can cause:

• Headache.
• High fever.
• Neck stiffness.
• Impaired consciousness, extreme lethargy, and reduced reactivity to outside stimuli (stupor).
• Disorientation.
• Coma.
• Tremors.
• Convulsions.
• Muscle weakness.
• Paralysis.

Many of these symptoms can be caused by other health problems. Always see your doctor for a diagnosis.

Certain things can increase the risk of getting West Nile virus. You are more likely to get the virus if you are exposed to mosquito bites during the summer months.

Most people who are infected have a minor illness and recover fully. But older people and those with weak immune systems are more likely to get a serious illness from the infection.

Your doctor will order a blood test to check for antibodies to the West Nile virus. They may also do a lumbar puncture (spinal tap) to test cerebrospinal fluid for signs of infection.

Treatment will depend on your symptoms, age, and general health. It will also depend on how bad the condition is.

There's no specific treatment for West Nile virus-related diseases. If a person gets the more severe form of the disease, West Nile encephalitis or meningitis, treatment may include intensive supportive therapy, such as:

• A hospital stay.
• I.V. (intravenous) fluids.
• A breathing machine (ventilator).
• The prevention of other infections, such as pneumonia or urinary tract infections.
• Nursing care.

Usually, the West Nile virus causes no symptoms or only mild, flu-like symptoms. But the virus can cause life-threatening illnesses, such as

• Inflammation of the brain (encephalitis).
• Inflammation of the lining of the brain and spinal cord (meningitis).
• Inflammation of the brain and its surrounding membrane (meningoencephalitis).

At this time, there's no vaccine available to prevent West Nile virus. The CDC recommends taking these steps to prevent mosquito bites and West Nile virus:

• Use EPA-registered insect repellent when you're outdoors. These include DEET (N,N-diethyl-meta-toluamide), picaridin, IR3535, oil of lemon eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone. If you spray your clothing, there's no need to spray repellent on the skin under your clothing.
• When possible, wear long-sleeved shirts and long pants treated with repellents containing permethrin, DEET, or another EPA-approved product for clothes, since mosquitoes may bite through thin clothing. Don't directly apply repellents containing permethrin to exposed skin.
• Think about staying indoors at dawn, dusk, and in the early evening. These are peak hours for mosquito bites, especially those mosquitoes that carry the West Nile virus.
• Limit the number of places for mosquitoes to lay their eggs. To do this, get rid of standing water sources from around your home.

Mosquitoes are drawn to people's skin odors and the carbon dioxide you breathe out. Repellents work by helping to prevent mosquitoes from biting you. But they only work at short distances from the treated surface. So mosquitoes may still be flying nearby. Always follow the directions on the insect repellent to find out how often you need to reapply repellent. To boost your protection from insect repellent, remember:

• Sweating or water may call for reapplication of the product.
• If you aren't being bitten, you don't need to reapply repellent.
• Use enough repellent to cover exposed skin or clothing. Don't apply repellent to skin that's under clothing. Heavy application isn't needed for protection.
• Don't apply repellent to cuts, wounds, or irritated skin.
• After returning indoors, wash treated skin with soap and water.
• Don't spray aerosol or pump products in enclosed areas.
• Don't apply aerosol or pump products directly to your face. Spray your hands and then rub them carefully over the face, staying away from your eyes and mouth.

Repellents vary on lengths of protection. Read the directions to find out how long your product will last.

Use care when putting insect repellent on children:

• Use an EPA-approved product for children. Use products with the lowest concentration of DEET, and no more than 30%. Do not use products with OLE or PMD on children under 3 years old.
• Repellent is not advised for infants under 2 months of age.
• When using repellent on a child, apply it to your own hands first. Then rub them on your child.
• Keep the repellant away from a child's eyes and mouth. Use the repellent sparingly around their ears.
• Don't apply repellent to children's hands. This is because children tend to put their hands in their mouths.
• Don't allow a young child to apply their own insect repellent.
• Keep repellents out of reach of children.
• Don't apply repellent to skin under clothing. If repellent is applied to clothing, wash treated clothing before wearing again.

Always contact your doctor for more information.

Most people infected with the West Nile virus will have no symptoms or only mild symptoms. But if any of these serious symptoms occur, get medical care right away:

• High fever
• Severe headache
• Stiff neck
• Confusion
• Muscle weakness
• Vision loss
• Numbness
• Paralysis
• Tremors
• Seizures
• Coma

• Humans get West Nile from the bite of an infected mosquito.
• Usually, the West Nile virus causes no symptoms or only mild, flu-like symptoms.
• The virus can cause life-limiting illnesses, such as encephalitis, meningitis, or meningoencephalitis.
• There is no vaccine available to prevent West Nile virus. So it's important to prevent mosquito bites.

Tips to help you get the most from a visit to your doctor:

• Know the reason for your visit and what you want to happen.
• Before your visit, write down questions you want answered.
• Bring someone with you to help you ask questions and remember what your doctor tells you.
• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your doctor gives you.
• Know why a new medicine or treatment is prescribed and how it will help you. Also know what the side effects are and when they should be reported.
• Ask if your condition can be treated in other ways.
• Know why a test or procedure is recommended and what the results could mean.
• Know what to expect if you do not take the medicine or have the test or procedure.
• If you have a follow-up appointment, write down the date, time, and purpose for that visit.
• Know how you can contact your doctor if you have questions, especially after office hours or on weekends and holidays.