West Nile Virus (WNV) is a virus that belongs to the genus Flavivirus.  It causes an infectious disease called "West Nile virus disease" or just "West Nile virus."  WNV mainly infects birds, but it can also infect humans, horses, dogs, bats, cats, reptiles, and amphibians.

West Nile virus is spread by mosquitoes, who get the virus from birds.  If a mosquito bites a bird that has WNV, and then bites a human, that person can get West Nile Virus.

West Nile virus was first discovered in 1937, in the West Nile area of Uganda, in East Africa.  (This is how the virus got its name.)  However, before the 1990s, there were very few cases of WNV.  Then there was an outbreak in Algeria in 1994 and another in Romania in 1996.  By 2004, the virus had spread to North America, the Caribbean islands, and Latin America.  It continues to spread through Africa, Asia, Australia, Europe, the Middle East, Canada, and the United States.  In 2012, one of the worst West Nile virus epidemics yet happened in the United States; 286 people died.

Signs and symptoms 
About 80% of people who get West Nile virus have no symptoms.

When a person does get symptoms, they usually do not appear until 2 to 15 days after the person got the virus.  (This is called the virus's incubation period.)

West Nile virus can cause many different types of illness.  The rarest, but most serious types affect the neurological system - the brain and the nerves.  The most common type of illness caused by WNV is West Nile fever, which does not affect the neurological system.

West Nile fever 
West Nile fever happens in about 20% of people with WNV.  It causes a high fever and flu-like symptoms.  Usually, the symptoms are not too bad, and get better after 3 to 6 days.  Symptoms may include:
 Headache
 Feeling weak and tired
 Pain in the joints
 Nausea, vomiting, and diarrhea
 Rash

West Nile neuroinvasive disease 
Neuroinvasive disease is when the West Nile virus infects the central nervous system (the brain and spinal cord).  This happens in less than 1% of people who get West Nile virus.  However, in those people, it can cause very serious problems.  These problems include:
 West Nile encephalitis: Encephalitis is swelling in the brain.  This can cause confusion, muscle weakness, paralysis, and loss of reflexes.  This is the most common type of West Nile neuroinvasive disease.
 West Nile meningitis: Meningitis is inflammation of the meninges, the layers that protect the brain and spinal cord.  People with meningitis usually have a bad headache, a high fever, and a stiff neck.
 West Nile meningoencephalitis: This is inflammation of both the brain and the meninges (encephalitis and meningitis).
 West Nile poliomyelitis: This causes the arms and legs on one side of the body to suddenly become weak or paralyzed.  The person may not have any other symptoms of WNV infection.  Sometimes, the breathing muscles can become paralyzed, and the person becomes unable to breathe.
 West Nile reversible paralysis: This also causes weakness or paralysis on one side of the body, but it almost always gets better.

Other complications 
WNV can cause some other complications that do not affect the neurological system.  These complications are very uncommon.  They include:
 Sudden hepatitis and liver damage
 Rhabdomyolysis (muscle death)
 Cardiac arrhythmia (when the heart does not beat normally)
 Hemorrhagic fever with coagulopathy (a high fever with severe bleeding, which the body is unable to stop by clotting the blood).  The bleeding can kill a person.
 Inflammation of the:
 Heart muscle (myocarditis)
 Pancreas (pancreatitis)
 Testes (orchitis)
 Kidneys (nephritis)
 Optic nerve (optic neuritis)

Diagnosis
At first, a medical professional usually makes a possible diagnosis of West Nile virus based on a person's symptoms, where they live, and when and where they have traveled.  The World Health Organization says that doctors should suspect West Nile virus if a patient:
 Has a fever that started suddenly; and
 Has neurological symptoms; and
 Was recently bitten by mosquitoes.

West Nile virus can be diagnosed for certain by:
 Doing blood tests that look for the antibodies that the body makes to fight the West Nile virus (like Immunoglobulin M, or IgM)
 Testing cerebrospinal fluid by doing a lumbar puncture (spinal tap)

Treatment
There is no known treatment or cure for WNV.  There are no known anti-viral medications (medicines which kill viruses) which will kill the West Nile virus.  Doctors can only provide "supportive care," which means they can only treat the symptoms of WNV.  For example, they can give fluids through a needle placed into a vein if a person is very dehydrated; help the person breathe if needed; and give medications to prevent them from getting any other infections.

Transmission
The West Nile virus stays alive in nature by getting spread back and forth between birds and mosquitoes.  Birds get WNV more often than any other living things.  When a mosquito bites a bird that has WNV, the mosquito gets the virus.  Then, if the mosquito bites a bird that does not have WNV, the mosquito can give that bird the virus.  In this way, birds and mosquitoes give the virus to each other and keep it alive.

Humans usually get West Nile virus by being bitten by mosquitoes who have gotten the virus from birds.  Humans can get the virus from many different species of mosquitoes.

Though this is not common, a person with West Nile virus can give another person the virus.  This can happen if:
 Blood infected with WNV gets into a person's body
 A person gets a blood transfusion or an organ transplant from someone with WNV
 A woman with WNV is pregnant (she can give the virus to her fetus)
 A woman with WNV breastfeeds her baby

Prevention

In contrast to the 4 currently available West Nile Virus vaccines for horses, there is no human vaccine available to prevent West Nile virus infection.  Six vaccines have been trialled in humans with the most successful only proceeding to Phase II trials. The best way for people to protect themselves against West Nile virus is to protect themselves against mosquitoes.  There are many ways to do this.  For example:
 Using bug spray, especially the type that has DEET in it
 Wearing long sleeves, long pants, socks, and hats
 Using a mosquito net over the bed when sleeping
 Putting bug spray on clothes, shoes, mosquito nets, bed sheets, and other things
 Staying in places that are air-conditioned or have good window and door screens
 Emptying out standing water (water that does not move) from things like buckets, gutters, and flowerpots (mosquitoes like to lay eggs in standing water)
 Being extra careful between evening and morning, when the mosquitoes that spread West Nile virus are most active

Scientists can trap mosquitoes and birds, and test them for West Nile virus to see if the virus is spreading in a certain area.  If it is, they can do things like spray pesticides to kill mosquitoes.

Related pages
 Flavivirus
 Infectious disease

Other websites

About West Nile virus
 West Nile Information for Seniors (Simple English)
 West Nile Virus - U.S. Centers for Disease Control and Prevention (CDC) page
 Nature news article on West Nile paralysis

Prevention
 Mosquito Bite Prevention (United States) (Simple English)
 Recommendations for Protecting Outdoor Workers from West Nile Virus Exposure
 Recommendations for Protecting Laboratory, Field, and Clinical Workers from West Nile Virus Exposure
 West Nile Virus Resource Guide—National Pesticide Information Center
 Vaccine Research Center (VRC) —Information about WNV vaccine research studies

For medical professionals

 Gene mutation turned West Nile virus into killer disease among crows 
 West Nile Virus Genomes  database search results from the  Viral Bioinformatics Resource Center

References

Diseases caused by viruses
Diseases spread by insects
Viruses