Right now, influenza is the most widespread illness circulating in the United States, with a test positivity rate of 17.9% as of late February 2026. RSV (respiratory syncytial virus) follows at 8.6%, and COVID-19 sits at 4.3%. Norovirus, the leading cause of stomach bugs, is also active this season, though outbreak numbers are running well below last year’s levels. Here’s what you need to know about each one.
Flu Is Dominating This Season
Influenza A is the main player, with the H3N2 strain accounting for most cases. H1N1 is also circulating but in smaller numbers. Influenza B cases remain relatively uncommon. Flu symptoms tend to hit fast: fever, cough, muscle aches, headache, and fatigue are all typical. A runny or stuffy nose is common with the flu in a way it often isn’t with COVID-19.
The incubation period for influenza A is short, roughly 1.4 days on average. Most people develop symptoms within 3 days of exposure, and some feel sick in under a day. Influenza B moves even faster, with a median incubation of just half a day. That quick turnaround is why flu can tear through a household or workplace so rapidly.
This season’s flu vaccine provides moderate protection overall, around 41% effectiveness against confirmed cases. Protection is strongest in young children: about 71% for kids 5 and under and 53% for those aged 6 to 17. For adults 18 to 59, effectiveness drops to around 25 to 45%, and it’s lowest in adults over 60. The reason for the gap is that the dominant H3N2 subclade (known as subclade K) doesn’t perfectly match the vaccine strain. Even so, vaccination still reduces the risk of severe illness across all age groups.
COVID-19 Is Still Circulating at Low Levels
With a positivity rate of 4.3%, COVID-19 is present but far less widespread than flu right now. The variant landscape has fragmented into several lineages. The XFG family of variants accounts for the largest share of cases, at roughly 29% combined, with several sub-lineages splitting the rest. No single variant is overwhelmingly dominant the way earlier strains like Delta or Omicron were.
The hallmark COVID symptom that still helps distinguish it from flu and colds is a loss of taste or smell without a stuffy nose. Common symptoms include fever, dry cough, shortness of breath, and fatigue. Sore throat, headache, and diarrhea show up less frequently. If your nose is congested and you’re sneezing a lot, a cold is more likely than COVID.
Home rapid antigen tests for COVID catch about 80% of cases that are actively contagious (culture-positive), but only about 47% of all PCR-positive infections. In practical terms, a positive home test is reliable, but a negative result doesn’t rule COVID out, especially in the first day or two of symptoms. Testing again 24 to 48 hours later improves accuracy.
RSV: Mostly a Concern for Young Children and Older Adults
RSV is running at an 8.6% positivity rate. For most healthy adults, RSV feels like a bad cold. For infants, young children, and older adults, it can cause more serious lower respiratory infections like bronchiolitis or pneumonia.
One shift worth noting: since the pandemic, RSV has been infecting slightly older children than it used to. Before COVID, about 24% of RSV cases in kids under 5 were in the 2-to-5 age group. By 2023, that figure had risen to 52%. Researchers believe reduced exposure during lockdown years left a larger pool of children without prior immunity, pushing the age of first infection upward. The upside is that hospitalization rates among children under 2 have actually declined, from 82% of cases requiring hospital stays in 2018-2019 down to 68% more recently. RSV seasonal peaks have also become less predictable, sometimes arriving two to three months later than the traditional winter window.
Norovirus: The Stomach Bug
Norovirus is the illness people often call “stomach flu,” though it has nothing to do with influenza. Symptoms include sudden vomiting, watery diarrhea, nausea, and stomach cramps, sometimes with a low-grade fever. It spreads incredibly easily through contaminated food, surfaces, and close contact with sick people.
This season, norovirus outbreaks are tracking within normal historical ranges. Between August 2025 and early February 2026, reporting states logged 644 outbreaks, compared to 1,707 during the same window last year. That drop sounds dramatic, but last season ran unusually high. Current numbers fall squarely in the middle 50% of what’s been seen over the past 12 years. Norovirus hits fast (symptoms usually begin 12 to 48 hours after exposure) and resolves within 1 to 3 days for most people. The biggest risk is dehydration, particularly in young children and older adults.
How to Tell What You Have
The overlap in symptoms between these illnesses makes self-diagnosis tricky, but a few patterns can help narrow things down:
- Sneezing and stuffy nose with mild symptoms: Likely a common cold. Fever is rare or low-grade.
- Sudden high fever, body aches, and exhaustion: Flu is the most probable cause right now given its high circulation. Symptoms come on fast and feel severe.
- Dry cough, fatigue, and loss of taste or smell: More suggestive of COVID-19, especially if nasal congestion is absent.
- Wheezing or trouble breathing in a young child: RSV should be considered, particularly in babies and toddlers.
- Vomiting and diarrhea with little or no respiratory symptoms: Norovirus or another stomach virus. Respiratory illnesses occasionally cause GI symptoms, but vomiting as the primary complaint points toward a stomach bug.
A home COVID test can confirm or help rule out SARS-CoV-2. Combination flu/COVID rapid tests are also available at most pharmacies. For RSV, testing is typically done in a clinical setting, most often for young children or hospitalized patients.
Warning Signs That Need Immediate Attention
Most people recover from these illnesses at home with rest and fluids. But certain symptoms signal that something more serious is developing.
In adults, seek emergency care for difficulty breathing, persistent chest or abdominal pain, confusion or difficulty staying awake, seizures, not urinating, or severe weakness. A fever or cough that gets better and then suddenly worsens is also a red flag, as it can indicate a secondary bacterial infection like pneumonia.
In children, watch for fast breathing or visible rib pulling with each breath, bluish lips or face, refusal to walk due to muscle pain, signs of dehydration (no urine for 8 hours, dry mouth, no tears), lack of alertness, or a fever above 104°F that doesn’t respond to fever reducers. Any fever in a baby under 12 weeks old warrants immediate medical evaluation, regardless of other symptoms.