People are getting sick with mystery illnesses and testing negative for COVID, RSV, and flu. Here's why.

• A lot of people are sick in the US right now with fevers, coughs, and sore throats.
• Flu and cold season has arrived early — and it's caught people by surprise.

It's the most wonderful time of the year ... to be a respiratory disease.

"The cold weather, the gathering indoors, all of that is good for respiratory viruses, and bad for symptoms," Centers for Disease Control and Prevention Director Rochelle Walensky said during a media briefing on Monday, stressing that multiple respiratory illnesses are here early this holiday season, challenging overstretched hospitals across the country.

Flu cases and hospitalizations have soared since Thanksgiving, but some people are complaining that they're testing negative for the flu, RSV, COVID, while still extremely sick.

Writer Cora Harrington said on Twitter that she got some "weird as hell virus" that made her "basically unconscious for a couple days," calling it "one of the strangest illnesses I've ever had."

General practitioner Stephanie de Giorgio from the UK said, similarly, that some kind of "not-flu, not-covid, not-RSV thing" was going around her workplace, and "felt bloody awful," prompting a fever and sore throat.

Doctors from at least three continents say that many different viruses — not just flu and COVID — are having a real "party" this year. People also shouldn't discount the idea that a COVID, RSV, or flu test taken early on may not necessarily go positive. Here's what to consider if you're feeling feverish right now.

Winter illness season is off to an early start

To give you a sense of how many sick people there are in the US, here is what the CDC's weekly "influenza-like illness" map — which tracks how many people are showing up at doctor's offices with fevers and coughs or sore throats — looked like on Thanksgiving week in 2021:

Here is Thanksgiving week 2022:

This red-hot level of "influenza-like illness" is a barometer that's based on patient symptoms (not viral tests) so it likely encompasses many cases of flu, COVID, and several other respiratory diseases showing up on doctors' doorsteps.

Trying to tell whether you've got the flu or COVID based on your symptoms? Good luck.

"Fever, muscle aches, cough, headache, those are going to be common," Dr. Roy Gulick, chief of infectious disease at New York-Presbyterian and Weill Cornell Medicine, told Insider. "You really can't tell the difference between flu and COVID."

This year's spike in respiratory illnesses has also arrived earlier than usual, and Gulick says it's "caught people by surprise" who haven't gotten their flu shot or COVID booster. But it's not too late to roll up your sleeves, if you haven't yet.

"There is still time to get vaccinated," Walensky said.

Flu season typically peaks in the US some time between December and February, but the illness can circulate well into May, or even later on in the spring and summer, as it did last season.

If you've got a fever, getting tested for flu and COVID can help you access treatment

If you have a fever and a sore throat or cough, experts like Gulick suggest getting tested for two respiratory viruses: flu and COVID. If you test postitive for either one, antiviral drugs may help you recover faster and safer from your infection, especially if you're elderly or in a high-risk group.

If you test negative for COVID and the flu, it's possible you may need to wait a few days and test again. But you could also have something else, like "the older coronaviruses, a virus called adenovirus, and of course just the common cold," Gulick said. All of those can cause an upper respiratory infection too. Other illnesses that are circulating now include metapneumovirus and parainfluenza.

"Most people probably don't need to know what they have," Gulick said. "They're gonna just go through the illness, and then get better with supportive care." Antibiotics are of no help for viral illnesses, so plenty of bedrest, fluids, and Tylenol or ibuprofen for fevers and aches is the tried and true method for at-home care.

While there is no treatment or vaccine available yet for viral illnesses like RSV, both flu and COVID are "ones that people can do something about," Gulick said, by getting vaccinated, boosted, and using antiviral medications like Paxlovid for COVID and Tamiflu for flu when they're warranted.

"We've always known cold and flu season is more than just flu," Walensky said. For influenza and COVID, "we want to make sure people know that there are prevention and treatment interventions."

Last week, nearly one in ten deaths nationwide was due to influenza, COVID, or pneumonia. Tragically, experts say, many of those deaths were preventable.