Sick season in the United States runs from roughly October through March, with most respiratory illness peaking between December and February. This window is when influenza, RSV, COVID-19, and the common cold all circulate at their highest levels, often overlapping in ways that make winter feel like one long stretch of illness.
When Each Virus Peaks
The major respiratory viruses follow slightly different calendars, but they all intensify during the same cold-weather months. Influenza is the most well-tracked: over a 40-year period, flu activity peaked in February more often than any other month (17 out of 40 seasons), followed by December (7 seasons), January (6 seasons), and March (6 seasons). Flu viruses are detectable year-round, but meaningful transmission clusters in fall and winter.
RSV generally starts circulating in early fall and peaks in winter, slightly ahead of influenza in many years. It hits hardest in infants and older adults, and its timing means it often fills pediatric hospitals before flu season has fully ramped up.
COVID-19 has settled into a loose seasonal rhythm. Based on the last four fall and winter seasons, the winter wave of COVID-19 typically peaks in December or January across all U.S. regions. That said, COVID-19 also circulates at meaningful levels throughout the year, with summer waves that don’t follow the same pattern as flu or RSV. The timing and size of each winter peak depend heavily on whether a new immune-evading variant emerges.
The common cold, most often caused by rhinoviruses, is the least seasonal of the group. Rhinoviruses are present on roughly 85% of days throughout the year. You can catch a cold in July. But cold frequency still ticks upward in fall and winter, partly because of the same environmental and behavioral factors that drive flu and RSV.
Why Winter Makes You Sick
The single strongest environmental predictor of flu season onset is a drop in absolute humidity, the total amount of moisture in the air. In temperate climates, absolute humidity bottoms out in winter and peaks in summer. Research published in PLOS Biology found that anomalously low humidity levels typically appear about two to four weeks before a flu epidemic begins, with the strongest signal roughly 17 days before onset. When the air is drier, flu viruses survive longer in airborne droplets and transmit more efficiently between people. Laboratory experiments with guinea pigs confirmed this directly: transmission rates climb sharply as humidity drops.
Cold temperatures reinforce the effect. Temperature and humidity are tightly correlated, and cold air masses sweeping down from the north bring both dry air and clear skies. Lower temperatures also appear to independently favor virus survival, though humidity is the more powerful factor.
Then there’s what you do differently in winter. You spend more time indoors, in closer proximity to other people, in spaces with less ventilation. Crowded indoor environments allow viruses to accumulate in the air and on surfaces. Schools, offices, and holiday gatherings all create ideal conditions for respiratory viruses to jump from person to person. This behavioral shift compounds the physical advantages viruses already gain from cold, dry air.
Vitamin D and Your Immune System
Your body produces vitamin D when your skin is exposed to UVB sunlight. In temperate climates, the total monthly sunlight drops dramatically in winter. One study measured roughly 103 hours of sunlight per month in winter compared to 240 hours in summer. The result: median blood levels of vitamin D doubled from winter to summer (43 to 89 nmol/L).
This matters because vitamin D helps regulate your immune response. When vitamin D levels are higher in summer, the body dials down inflammatory immune signaling, which sounds counterintuitive but actually reflects a well-tuned system. In winter, lower vitamin D levels are associated with a more reactive, less regulated immune response, and epidemiological studies have found an inverse relationship between vitamin D levels and the incidence of upper respiratory infections. People with lower vitamin D get more colds and flu. This seasonal vitamin D dip is one reason populations at higher latitudes tend to see more pronounced sick seasons.
Telling the Viruses Apart
Flu, RSV, COVID-19, and the common cold produce overlapping symptoms: cough, congestion, fatigue, and sometimes fever. This makes it genuinely difficult to tell them apart based on how you feel. A few general patterns can help, though none are reliable enough to replace a test.
- Influenza tends to hit suddenly, with body aches, high fever, and exhaustion that come on within hours rather than building over days.
- RSV in adults often looks like a bad cold, with wheezing and cough. In infants and young children, it can cause rapid, labored breathing.
- COVID-19 is known for loss of taste or smell (though this has become less common with newer variants), and its symptoms can persist or fluctuate over a longer period.
- Common colds are the mildest of the group, centered on a runny nose, sneezing, and sore throat, usually without significant fever.
Because the symptoms overlap so much, diagnostic testing is the only reliable way to confirm which virus you have. Combination tests that check for flu, COVID-19, and RSV simultaneously are widely available at pharmacies and clinics.
Best Timing for Flu Vaccination
The CDC recommends getting your flu shot in September or October for most people. This timing builds peak immunity before the December-through-February danger zone. Vaccination earlier than September, during July or August, is generally discouraged for adults, especially those 65 and older, because protection can wane before the season peaks. The exception is children aged 6 months through 8 years who need two doses: they should get their first dose as soon as vaccine becomes available, since the second dose follows at least four weeks later.
Getting vaccinated later in the season still helps. Flu viruses circulate well into March and sometimes April, so a January or February shot is better than skipping it entirely.
What the Southern Hemisphere Signals
Australia and New Zealand experience their flu season during the Northern Hemisphere’s summer, roughly May through September. Public health officials watch these seasons closely because the strains that dominate in the Southern Hemisphere often preview what’s coming north. In 2025, Australia and New Zealand experienced extended flu seasons driven by a new H3N2 subgroup (called subclade K viruses) that spread rapidly and has already been detected in over 34 countries. These viruses are antigenically distinct from recent strains, meaning the current Northern Hemisphere vaccine component may offer reduced protection if subclade K viruses circulate widely during the 2025-2026 winter. This could translate into more cases and hospitalizations compared to recent seasons.
A Practical Sick Season Timeline
If you want to think of sick season as a calendar, here’s how it typically unfolds in the United States:
- September-October: RSV begins circulating. This is the ideal window for flu vaccination.
- November: Flu activity picks up. Early cold and RSV cases are climbing.
- December-February: The peak months for flu, RSV, and COVID-19 winter waves. February is the single most common month for flu to peak.
- March-April: Activity tapers off for most viruses, though late-season flu waves occasionally push into spring.
The exact timing shifts from year to year. New variants, unusual weather patterns, and population immunity levels all influence when each virus surges. But the broad pattern holds remarkably steady: cold, dry air arrives, people move indoors, vitamin D levels drop, and respiratory viruses thrive.