Despite having first appeared in the United States only 16 years ago, West Nile virus holds the dubious title of most common mosquito-borne virus in the country, according to the U.S. Centers for Disease Control and Prevention. The only good news is that it is still relatively rare and that the number of known infections is holding steady.
It was first diagnosed in the United States in New York City, but has now spread to the 48 contiguous states and to Canada, the CDC said in Morbidity and Mortality Weekly Report. There were 2,205 reported cases of West Nile in 2014, which is almost all of the 2,327 reported cases of mosquito-transmitted viral disease.
For most people, West Nile infection does not present a problem. About three out of four people infected with it develop no symptoms, according to the CDC. Those who do get ill usually feel like they have a bad case of the flu, with fever muscle aches, and a headache. But a bit less than 1% of infections develop into encephalitis (an inflammation of the brain) or meningitis (the membranes around the brain). Symptoms of these neuroinvasive infections include headache, high fever, a stiff neck, coma, seizures, or paralysis, according to the CDC.
In 2014, nearly 1,300 people were hospitalized with the virus. There were more than 600 cases of West Nile that involved encephalitis and over 500 that involved meningitis, the CDC researchers found.
West Nile is only caught through a mosquito bite. It cannot be transmitted from one person to another except through a blood transfusion.
There is no treatment for West Nile virus outside of drugs like aspirin that relieve the symptoms.
Other mosquito-borne illnesses in the United States in 2014 were La Crosse virus, Jamestown Canyon virus, St. Louis encephalitis virus, Powassan virus, and eastern equine encephalitis virus.
You can read the CDC's report here.