West Nile hit Dr. Fauci like a 'truck.' Here's what to know about the virus.

As the U.S. enters peak mosquito season, a prominent figure in public health said his bout with West Nile virus felt like he'd “been hit by a truck.”

Dr. Anthony Fauci, the former director of the National Institute of Allergy and Infectious Diseases and a leader of the nation’s early response to the COVID-19 pandemic, made this comment after spending several days hospitalized for West Nile , a mosquito-borne virus.

On Saturday, Fauci's spokesperson told news outlets, including USA TODAY, that the 83-year-old immunologist contracted West Nile and had spent six days in the hospital. He has been recovering since.

In an interview with STAT , a health news site, Fauci said he felt like he’d “been hit by a truck.”

“I’ve never been as sick in my life. Ever,” he told the publication. “By far, this is the worst I’ve ever been with an illness.”

Fauci's was a rare case. Most people infected with West Nile by mosquitoes don’t develop symptoms, although rare cases can result in lasting neurological issues and even death.

When did West Nile emerge?

West Nile became endemic in the U.S. 25 years ago in Fauci’s hometown of New York City. There are now thousands of cases yearly, although experts say the incidents are vastly underreported.

So far in 2024, the U.S. has reported 216 cases – 142 involved people who developed the severe neuro-invasive form of the disease, according to the Centers for Disease Control and Prevention .

“We've learned a lot, but we still have the same tools available that we did back then to prevent diseases,” Dr. Erin Staples, a medical epidemiologist with the CDC’s vector-borne diseases division, told USA TODAY. “We need to do better.”

After all, she explained, mosquitoes are the world’s deadliest animal.

What is West Nile virus?

West Nile circulates mostly among birds that serve as vectors for mosquitoes, according to the CDC. West Nile was first identified in Uganda’s West Nile region before it spread across Africa, Europe, Asia, Oceania and the Americas, the European Centre for Disease Prevention and Control said. In the U.S., West Nile follows mosquito season beginning in the summer months. Infections typically are identified in August and September.

Mosquitoes, typically from the Culex genus, bite the birds, contract the disease and then spread it to people. People don’t develop a high enough level of the virus in their bloodstream for mosquitoes to contract it again and spread it to other humans. In rare cases, West Nile has been transmitted between people by blood transfusions, organ transplants, or from mothers to babies.

There is no human vaccine to prevent West Nile, but there is a vaccine for horses .

How common is West Nile?

West Nile is the leading cause of disease wrought by mosquitoes in the U.S., according to the CDC. In August 1999, the first year it was introduced to the U.S., there were 62 cases and a handful of deaths.

In subsequent years, the virus began to spread by migratory birds when they traveled across the country. The U.S. has seen a surge in human infections beginning with about 66 cases in 2001 and rising to more than 4,100 recorded cases in 2002. There were 284 deaths from West Nile that same year. In 2003, 9,862 people were infected and 264 died in the U.S. Cases are now highest in the central and western U.S.

West Nile cases are massively underreported, and many people are unaware they have the virus. The cases reported are typically the most severe infections. A blood test has traditionally been used to detect West Nile in patients, though samples of cerebrospinal fluid have been used to look for the neurologic form of disease, said Staples, of CDC.

What are the symptoms?

About 4 out of 5 people who contract West Nile never show symptoms. About 1 in 5 people infected develop the most common symptom, West Nile fever. Other common symptoms include headaches, body or muscle aches, joint pain, vomiting, diarrhea and rash, the CDC said. Symptoms usually occur 1 to 14 days after a person has been bitten and can last three to six days, according to Penn Medicine , a hospital and medical school.

In rare cases, a more severe neuro-invasive form develops that can cause encephalitis or meningitis. Symptoms include muscle weakness, stiff neck, weakness in an arm or leg, confusion, loss of ability to think clearly, and loss of consciousness or coma.

Around 10% of these severe cases result in death. In 2023, 182 people in the U.S. died of West Nile.

Anyone can develop severe illness, but older people and people with certain underlying conditions – including cancer, cardiovascular issues and kidney disease – and organ transplant recipients are at greater risk. Most people infected with West Nile become protected against future infections.

How to prevent West Nile

The best way to prevent West Nile is to limit the areas where mosquitoes can thrive and bite people. Here are some recommended steps:

• Use an Environmental Protection Agency-registered insect repellant .
• Wear loose-fitting, long-sleeve shirts and pants when outdoors.
• Remove areas where mosquitoes can populate, such as standing water where mosquitos can lay eggs, around homes. Use screens on windows and doors and repair holes in screens.
• If accessible, use air conditioning rather than leaving windows and doors open on hot days.

This article originally appeared on USA TODAY: West Nile hit Dr. Fauci like a 'truck.' Here's what to know about the virus.