This ‘Grossly Underreported’ Virus Is Often Mistaken for the Flu, Doctors Say

Korin MillerFri, October 16, 2026 at 3:07 PM UTC

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This Deadly Virus Is Often Confused With the FluRealPeopleGroup - Getty ImagesWest Nile virus causes symptoms similar to the flu, which can make them hard to differentiate. Here's what doctors want you to know.

When you feel run-down, feverish, and generally all-around lousy, it’s understandable to think you’re coming down with the flu—after all, cases are ramping up earlier than usual this year. But there’s another virus that causes similar symptoms that’s been circulating at higher-than-normal levels, too: West Nile virus.

You’re probably not as familiar with West Nile virus as the flu, since it’s not nearly as common. But the mosquito-borne virus is pervasive—45 states have reported a case of it this season—and it’s been causing an unusually high number of illnesses. The U.S. has recorded 1,328 cases of the virus so far this year, according to data from the Centers for Disease Control and Prevention (CDC). Of those, 1,004 were neuroinvasive, the most severe form of the infection that attacks the central nervous system.

And doctors say the actual number of West Nile cases is likely higher. “West Nile is grossly underreported,” says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. Most people with the virus have no symptoms at all, making it difficult for public health researchers to know exactly how widespread it is, he points out. “Most cases likely go under the radar,” Dr. Russo adds.

One complicating factor? When symptoms do occur, they often mimic signs of the flu—so people tend to treat it as such, meaning, many choose at-home care without seeking an actual diagnosis. But West Nile virus can be deadly—the North Carolina Department of Health and Human Services just announced that two people in the state have died from it.

Here’s what infectious disease doctors want you to know about the illness, including signs of the mosquito-borne virus to watch for—and what to do if you think you have it.

Meet the experts: William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine; infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security; Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York.

Why are West Nile cases higher this year?

Cases of West Nile virus show up in the U.S. every year, but there’s been a recent surge in numbers. One possible reason is because cases showed up earlier than usual this year—typically, West Nile virus “season” runs from June to October, peaking in late August to early September, according to the CDC. But according to its July 1 news release, the U.S. had already seen its highest numbers of cases by that time since 2004. And cases are still being reported now.

Because West Nile is transmitted by mosquitoes, weather plays a big role in case count.

“Heat is the main driver” of West Nile cases, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. “A milder winter and hot, humid conditions favor the vector mosquito.”

So it follows in line that the highest number of cases this year has been in California, Arizona, and Texas, although Colorado, North and South Dakota, Nebraska, and Utah have also seen plenty of cases, Dr. Adalja points out.

“Mosquitoes have been more common in areas of the country where it’s been a bit wetter than usual,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. “People in those areas should be on the alert.”

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West Nile virus symptoms

Up to 80% of people with West Nile virus do not develop symptoms of the infection, per the CDC. But 20% of those infected will develop flu-like symptoms. Those include:

Fever

Headache

Body aches

Joint pain

Vomiting

Diarrhea

Rash

Fatigue

The fatigue and weakness from West Nile virus can linger for weeks or months, according to the CDC.

How to tell West Nile virus from the flu

If that West Nile virus symptom list sounds familiar, it’s because there’s a big overlap there with flu signs. So differentiating West Nile virus from the flu can be tricky.

And while you may think linking your symptoms to a mosquito bite would be the best way to tease out the cause, that’s not necessarily the case.

“Most people never remember the bite, so the pattern of symptoms matters more,” Dr. Adalja says.

What can be more helpful is considering what you don’t have: He suggests suspecting West Nile virus if you have a fever, headache, and body aches—sometimes with a rash, nausea, or diarrhea—but without the cough, congestion, or sore throat that’s more common with respiratory viruses like the flu.

There is a test for West Nile virus, Dr. Russo points out. “The turnaround time is quite variable, and convincing your doctor to give it to you may be tough,” he says. “But the lack of respiratory symptoms will differentiate it from things like flu and COVID.”

What to do if you suspect you have West Nile virus

Start with the process of elimination. Dr. Schaffner recommends testing for the flu and COVID-19 first. “Since flu is early this year, it’s more likely to be flu, although it could be COVID,” he says. If both are ruled out, he says it’s worth asking your provider if there’s a possibility that you have West Nile virus, especially if you know you were recently bitten by a mosquito.

Unfortunately, there is no treatment available for West Nile virus. “There’s not much you can do—treatment is really supportive,” Dr. Russo says. Doctors will usually recommend that you try to rest, get plenty of fluids, and take acetaminophen or ibuprofen for fever and aches, according to Dr. Adalja.

Ultimately, it’s best to do what you can to avoid being bitten by mosquitoes. “The best strategy is avoiding mosquitoes with repellent, removing standing water around homes, and keeping mosquitoes out of your house,” Dr. Adalja says. And you can also minimize your chances of coming down with the viruses that can cause similar symptoms, too—don’t forget vaccines for both flu and COVID-19 can cut down your chances of developing a severe case of the illness or getting sick in the first place.

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