This winter, it feels like illnesses are everywhere, and you’re not wrong. Health experts in the U.S. have identified a significant surge of four common viruses – creating what they’re calling a “quad-demic.” Data from the Centers for Disease Control and Prevention (CDC) confirms that COVID-19, influenza (the flu), respiratory syncytial virus (RSV), and norovirus are all circulating at high levels across the country. While the increased spread of respiratory illnesses during the colder months isn’t unusual, understanding what’s going around and how to protect yourself is crucial.
The “Quad-demic” Viruses: Respiratory Illnesses Leading the Charge
It’s typical to see an uptick in respiratory illnesses like COVID-19, the flu, and RSV during the winter season. According to Dr. Shira Doron, chief infection control officer for Tufts Medicine, current infection rates for these viruses are within the expected range for this time of year. Several factors contribute to this winter surge, including environmental conditions (cold, dry air aids virus spread), potentially weakened immune systems during winter, and increased indoor gatherings.
Influenza is currently particularly widespread across the United States. The CDC’s Weekly US Influenza Surveillance Report indicates high levels of flu activity in most states, with many experiencing “high” or “very high” levels.
While still present, COVID-19 and RSV are currently circulating at lower levels compared to the flu. For the week ending January 4th, COVID-19 was responsible for 1.3% of emergency department (ED) visits nationwide, and RSV accounted for just 0.9%. In contrast, the flu was the cause of 4.8% of ED visits during the same period.
Dr. Doron suggests that RSV may have already peaked for this season, and COVID-19 rates seem to have stabilized. She anticipates that flu cases will likely begin to decline in February.
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Norovirus: The Stomach Bug on the Rise
Adding to the mix, norovirus, often referred to as the “stomach flu,” is also actively spreading. Although not a respiratory illness, norovirus outbreaks are more frequent this season than usual.
“Norovirus is having a relatively high year,” notes Dr. Doron. CDC data confirms this trend. From August 1st to December 11th, 2024, the U.S. reported 495 norovirus outbreaks – exceeding the number reported during the same period in 2023 by over 100. In fact, the outbreak count for the 2024–2025 season has already surpassed the total outbreaks reported for each season since 2012.
This data might even underestimate the true extent of norovirus circulation, as many individuals with norovirus symptoms don’t seek medical attention. Dr. Doron explains that testing for norovirus isn’t typically prioritized because there’s no specific treatment. Management focuses on symptom relief and hydration, as norovirus is the most common cause of nausea, vomiting, and diarrhea.
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Beyond the Usual Suspects: Other Illnesses to Consider
While the “quad-demic” viruses are prominent, experts are also keeping an eye on other illnesses this season, including whooping cough, walking pneumonia, and even bird flu.
Whooping cough (pertussis), a highly contagious bacterial infection, is showing a resurgence to pre-pandemic levels, according to the CDC. Recent data indicates that reported cases as of December 28, 2024, were more than six times higher compared to the same period in 2023.
Earlier in the season, in October, cases of walking pneumonia, a respiratory infection caused by Mycoplasma pneumoniae bacteria, were also elevated in the U.S.
Avian influenza, commonly known as bird flu, is another area of close monitoring. While still rare in humans, Dr. Doron emphasizes that it’s important to stay informed. To date, there have been 66 confirmed human cases of bird flu in the U.S., with one associated death linked to the H5N1 virus. Most cases have been linked to direct contact with infected animals, such as in dairy herds or poultry farms, but some cases have had unknown sources of exposure.
Human metapneumovirus (HMPV), a virus causing cold-like symptoms, has been in the news due to a surge in China. However, Dr. Doron reassures that Americans shouldn’t panic. She explains that China’s situation may be unique due to an “immunity debt” period following strict COVID-19 restrictions, leading to a simultaneous surge of various infections.
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Staying Healthy When Viruses Are Widespread
How you protect yourself during this season of heightened virus activity depends on your individual risk tolerance and health history.
Basic precautions like frequent handwashing and distancing from visibly ill individuals are always beneficial. Those at higher risk of severe illness, such as immunocompromised individuals, older adults, and young children, might consider additional protective measures. Wearing a mask in public indoor settings or limiting social interactions can provide extra layers of defense.
Maintaining a healthy lifestyle throughout the winter months is also crucial. Prioritizing a balanced diet, regular exercise, and sufficient sleep can strengthen your immune system.
Dr. Doron aptly points out, “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. The best way to protect yourself is to be really healthy when you do get a cold.” Being proactive about your health and taking reasonable precautions can help you navigate this season of widespread illnesses.
