As of late February 2026, the main illnesses circulating in the United States are seasonal influenza, COVID-19, RSV, norovirus (stomach bug), and measles. The overall level of respiratory illness sending people to seek medical care is moderate nationally, though activity varies widely by state. Here’s what’s spreading, how to tell these illnesses apart, and what to watch for.
Flu Is the Dominant Respiratory Illness Right Now
Influenza is driving more emergency department visits than any other single respiratory virus, accounting for 3.2% of all ED visits nationally for the week ending February 21, 2026. That’s roughly five times the rate of either COVID-19 or RSV. Influenza A activity is on the decline, but influenza B is picking up across most of the country, which is a typical late-season pattern.
States with the highest overall respiratory illness activity include Alabama and Arkansas, both rated “high.” Most of the country is sitting at low or moderate levels, with Maine, North Carolina, and Virginia at “very low.” If you’re in the South or parts of the Midwest, you’re more likely to encounter active transmission right now.
The current flu vaccine is showing roughly 56% effectiveness against circulating strains, based on preliminary CDC estimates. That means it cuts your risk of needing medical care for the flu by about half. Flu symptoms typically appear one to four days after exposure, and you’re most contagious during the first three days of illness.
COVID-19 Is Declining but Still Circulating
COVID-19 activity is decreasing nationally, with emergency department visits at 0.6%. However, wastewater surveillance tells a more detailed story: several states, including Arizona, Oklahoma, and Washington, are showing “very high” COVID-19 levels in wastewater, meaning the virus is still spreading actively in those communities even if fewer people are showing up at hospitals.
The dominant variants right now are from the XFG family. XFG alone makes up about 29% of sequenced cases, with related sublineages (XFG.2.5.1, XFG.1.1, XFG.14.1, and XFG.6) adding another 36%. NB.1.8.1 accounts for 21%. These variants produce the familiar set of symptoms: fever, cough, sore throat, fatigue, body aches, and congestion. Loss of taste or smell can still occur with COVID and remains more common with COVID than with flu.
One practical difference: COVID’s incubation period runs two to five days (sometimes up to 14), and you can spread it for about eight days after symptoms start. That’s a longer contagious window than the flu, which is why COVID still spreads efficiently even as case counts drop.
RSV Is Climbing in Parts of the Country
RSV (respiratory syncytial virus) is elevated and increasing in some regions. Like COVID, it accounts for about 0.6% of emergency department visits nationally, but the real concern with RSV is who it hits hardest: infants, toddlers, and older adults. In most healthy adults, RSV looks like a bad cold with congestion, cough, and low-grade fever. In young children, it can cause wheezing, difficulty breathing, and sometimes bronchiolitis or pneumonia.
RSV’s incubation period is about four to five days. Wastewater data shows “very high” RSV levels in Arizona, Oklahoma, and Washington, with moderate levels in Alabama and California.
Norovirus (Stomach Bug) Is at Typical Levels
If you or your kids are dealing with sudden vomiting and diarrhea, norovirus is the most likely culprit. Between August 2025 and early February 2026, 644 norovirus outbreaks were reported to the CDC by participating states. That’s within the normal historical range, though well below the 1,707 outbreaks reported during the same window last season.
Norovirus spreads incredibly easily through contaminated food, surfaces, and close contact. Symptoms hit fast, usually within 12 to 48 hours of exposure, and include intense nausea, vomiting, watery diarrhea, and stomach cramps. Most people recover in one to three days, but dehydration is the main risk, especially for young children and older adults. The best prevention is thorough handwashing with soap and water. Alcohol-based hand sanitizers are less effective against norovirus.
Measles Is Surging Beyond Recent Norms
The United States is experiencing an unusually large measles surge. As of February 26, 2026, there have been 1,136 confirmed cases across 28 states, and we’re only two months into the year. For context, all of 2024 saw just 285 cases. The full year of 2025 had 2,281 cases, and 2026 is already on pace to exceed that.
South Carolina has been hit particularly hard, reporting 653 cases. Utah (149 cases) and Arizona (42 cases) are also seeing significant outbreaks. Ninety percent of this year’s confirmed cases are linked to outbreaks rather than isolated infections, which signals sustained community transmission. Measles is one of the most contagious diseases that exists. It spreads through the air and can linger in a room for up to two hours after an infected person leaves. The incubation period is long, around 12 to 13 days, so you may not realize you’ve been exposed until well after the fact. Symptoms start with high fever, cough, runny nose, and red eyes, followed by a characteristic rash that spreads from the face downward. Two doses of the MMR vaccine are about 97% effective at preventing infection.
Walking Pneumonia in Children
Infections caused by the bacterium Mycoplasma pneumoniae, commonly called “walking pneumonia,” have been rising in children. CDC data from 2024 showed a notable jump in pneumonia-related emergency visits linked to this bacteria, with discharge diagnoses climbing from 1% to 7.2% among children ages 2 to 4, and from 3.6% to 7.4% among kids ages 5 to 17. The increase in the younger age group is unusual because this type of pneumonia hasn’t historically been a leading cause of illness in toddlers.
Walking pneumonia typically causes a persistent cough that can last for weeks, along with low-grade fever, fatigue, and a general feeling of being unwell. It’s called “walking” pneumonia because most kids (and adults) don’t feel sick enough to stay in bed, but the lingering cough can be disruptive. It’s treatable with antibiotics, unlike viral respiratory infections.
How to Tell These Illnesses Apart
The frustrating reality is that flu, COVID, and RSV share most of their symptoms: fever, cough, sore throat, body aches, fatigue, and congestion. There are a few distinguishing clues, though. Loss of taste or smell still points more toward COVID. Flu tends to hit suddenly and intensely, with high fever and severe body aches from the start. RSV in adults usually feels like a stubborn cold, heavier on the congestion and cough than the body aches.
Vomiting and diarrhea without significant respiratory symptoms suggest norovirus. A rash that starts on the face after several days of fever and cold-like symptoms is a hallmark of measles. And a dry cough that just won’t quit in a child who otherwise seems mostly okay is the classic pattern for walking pneumonia.
The only way to confirm which virus you have is testing. Many clinics now offer combination tests that check for flu, COVID, and RSV simultaneously. Home COVID tests remain widely available, and a positive result on one can save you the guesswork.
Signs That Need Immediate Attention
Most of these illnesses resolve on their own with rest and fluids, but certain symptoms signal something more serious. Seek emergency care for a fever above 103°F, difficulty breathing, chest pain, wheezing or noisy breathing, dizziness, confusion, or visible skin pulling inward between the ribs with each breath (called retractions, especially important to watch for in children). In infants, watch for refusal to eat, significantly fewer wet diapers, and any bluish tint around the lips or fingernails.