What’s Going Around Right Now? The 4 Winter Viruses of the ‘Quad-Demic,’ Explained

Four illnesses are circulating in full force this winter, creating what many experts are calling a “quad-demic” in the U.S.

The most recent data from the Centers for Disease Control and Prevention show that “the amount of acute respiratory illness causing people to seek healthcare is at a high level.” Rates of COVID-19, seasonal influenza (the flu), and respiratory syncytial virus (RSV) are elevated across most areas of the country.

Similarly, norovirus—known commonly as the stomach flu—has been on the rise in the U.S., causing outbreaks across the country. According to CDC NoroSTAT data, a record 91 norovirus outbreaks were recorded for the week ending Dec. 5, 2024—far more outbreaks than in recent years at this time.

These simultaneously spreading viruses aren’t only a burden to public health, but put added strain on healthcare systems.

“It [has] a huge impact, not just on the individual who’s sick, but on the community,” Geeta Sood, MD, ScM, assistant professor of medicine and hospital epidemiologist at Johns Hopkins Bayview Medical Center, told Health. “The entire healthcare system can be impacted when you have high rates of all of these infections at the same time.”

Here’s what you need to know about the four viruses of the quad-demic and how to stay safe this winter.

It’s not uncommon for respiratory illnesses, like COVID, the flu, and RSV, to spread more in the winter months, and current rates of infection are pretty typical for this time of year, according to Shira Doron, MD, chief infection control officer for Tufts Medicine and hospital epidemiologist at Tufts Medical Center.

This is due to a variety of reasons, like environmental factors (cold, dry conditions can allow viruses to spread more easily), more susceptible immune systems, and frequent indoor gatherings.

Influenza in particular is ravaging most of the U.S. right now. According to the CDC’s Weekly US Influenza Surveillance Report, seasonal flu activity is elevated across most of the country, with many states seeing high or very high levels of flu.

COVID and RSV are also circulating now, but at lower levels. For the week ending Jan. 4, COVID accounted for 1.3% of emergency department visits in the U.S., while RSV made up just 0.9%. (Flu, meanwhile, accounted for 4.8% of ED visits.)

Doron told Health that she believes RSV has hit its peak for this season, while COVID rates appear to have reached a plateau. She also predicted that rates of the flu will likely start to fall in February.

Though not a respiratory illness, norovirus is also circulating in the U.S., and has caused more outbreaks than usual this season.

“Norovirus is experiencing a relatively high year,” Doron said. “It’s not a terrible outlier, but it’s not a good year for norovirus.”

The most recent data from the CDC show that, from Aug. 1 to Dec. 11, 2024, the U.S. saw 495 norovirus outbreaks—over 100 more than the same time period from 2023. In fact, the total number of outbreaks for the 2024–2025 season so far has surpassed the outbreaks reported for all previous seasons since 2012.

Even that data may be underreported, said Doron, since many people don’t seek medical care for its symptoms.

“You can’t treat norovirus, so you’re not really trying very hard to find it,” Doron said, explaining the lack of testing. “It is the most common cause of nausea, vomiting, and diarrhea, so you’re just going to generally manage it symptomatically and with hydration.”

Experts say whooping cough, walking pneumonia, and even bird flu deserve some extra attention this season.

Whooping cough (pertussis), a highly contagious bacterial infection, is returning to pre-pandemic patterns, according to the CDC. The most recent data show more than six times as many cases were reported as of Dec. 28, 2024, compared to the same time in 2023.

Earlier this season, in October, the U.S. was also seeing an increase in cases of walking pneumonia, a respiratory illness caused by the Mycoplasma pneumoniae bacteria.

Many experts are watching avian influenza–commonly known as bird flu—closely as well.

Though bird flu is still very rare, according to Doron. There have been 66 confirmed total cases of bird flu in the U.S., and one known death associated with the H5N1 virus. Most bird flu cases have been directly linked to animal exposures, such as dairy herds or poultry farms, but at least two cases have an unknown source of exposure.

And though human metapneumovirus (HMPV)—a virus that has symptoms similar to the common cold—has been in the news recently for a surge of cases in China, Doron said there’s no reason for Americans to panic.

“We can’t assume that what China sees, we will see,” said Doron. “Because they’re still in that immunity debt period of having not seen anything for so long that everyone’s getting everything at the same time.”

The precautions you take during cold and flu (and norovirus) season depend on your own risk tolerance and medical history.

Some people may feel comfortable washing their hands a little more often and keeping their distance from people who are ill. Others who may be more susceptible to infections—like immunocompromised people, the very old, or the very young—may take an extra protective step and wear a mask or limit social interactions.

It’s also wise to stay as healthy as possible (eat well, exercise often, optimize sleep) throughout the winter months, just in case.

“These respiratory infections and norovirus are inevitable. Everyone’s going to come in contact with them. You can run, but you can’t hide from them,” Doron said. “The best way to protect yourself is to be really healthy when you do get a cold.”