What is West Nile virus?

West Nile virus is part of the flavivirus family, which includes St. Louis encephalitis virus, Japanese encephalitis virus, and Powassan virus.

The virus is most commonly spread through the bite of an infected mosquito. The bugs become infected after biting sick birds and then spread the virus to people through another bite.

"People are considered dead-end hosts because, unlike birds, they do not develop high enough levels of virus in their bloodstream and cannot pass the virus on to other biting mosquitoes," CDC said.

Most people who contract West Nile virus develop no symptoms at all; however, CDC notes that around one in five people can develop a fever, headache, body aches, vomiting, diarrhea, or a rash. Adults ages 60 and over are at a higher risk of developing serious disease if they're infected.

According to CDC, around one in 150 people will see the virus move to their nervous system, which can cause encephalitis or meningitis. Around 10% of people who develop this form of West Nile virus die. Last year, 182 deaths from West Nile virus were recorded in the United States.

Since its discovery in the United States in 1999, West Nile virus has become the most common mosquito-borne illness in the country. As of Aug. 20, CDC said it had recorded 216 West Nile virus cases in 33 states, and 142 of those patients experienced the neurological form of the disease.

'I really felt like I'd been hit by a truck'

Fauci, 83, was hospitalized for West Nile virus on Aug. 16 but is now recovering at home. Spokesperson Jenn Kuzmuk in a statement Sunday said that a "full recovery is expected."

Jonathan LaPook, CBS News' chief medical correspondent, said in a social media post that he spoke to Fauci on Saturday, who said he was likely infected by a mosquito bite in his backyard.

Fauci told STAT that he started feeling sick two weeks ago and assumed he was developing "a plain old upper respiratory infection." He said he initially felt "weak and worn out and tired … But then the weakness got really profound."

Fauci's temperature spiked to 103 degrees Fahrenheit, and for several nights, he said he experienced hours of "shaking chills" that wouldn't go away. Even after he was released from the hospital, Fauci said the illness left him so weak he couldn't initially stand without help.

"I really felt like I'd been hit by a truck," he said. "I have to tell you, I've never been as sick in my life. Ever. By far, this is the worst I've ever been with an illness."

Fauci added that he did not develop the neurological form of West Nile virus, and said he knows a full recovery will take time.

"I just have to rest," he said. "The good news is I'm absolutely going in the right direction. The sobering news is that it's going to take weeks and weeks to get back to normal. That's the history of West Nile. It just wipes you out so badly." (Thompson, Associated Press, 8/24; Diaz, New York Times, 8/25; Branswell, STAT, 8/24; Chile/Strauss, ABC News, 8/24; Edwards, NBC News, 8/24)