The Culex genus of mosquitoes has a worldwide distribution and is found on every continent except Antarctica. Fourteen species occur throughout the United States and Canada. Many species of Culex are known as the "house mosquito" because these insects commonly develop in small containers around houses. The main host for Culex is wild birds, but it also feeds on a wide variety of warm-blooded vertebrates, including humans.

View a second image of a Culex mosquito.

Culex is a carrier of viral encephalitis and, in tropical and subtropical climates, of filariasis. In the United States, they are disease vectors for western equine and Saint Louis encephalitis. They are also vectors for the West Nile virus that suddenly appeared in the Queens borough of New York City in the summer of 1999, killing six people. The virus, established in Africa and Europe, was most likely introduced into the United States by accident. When exactly this happened is subject to debate. The virus may have been in the states for years, but until people started dying from it, one knew about it. Given that Culex and birds are found everywhere there are people in North America, this disease is probably here to stay.