Flu season is one of the most reliable rhythms in public health: every winter, doctor visits for flu-like illness surge, then fade by spring. But no two seasons are alike — some are mild, some brutal — and one recent season nearly disappeared entirely. This guide charts more than 25 years of U.S. flu activity, explains the metric behind it, and shows how the pandemic briefly broke the flu's annual pattern.
What is 'influenza-like illness'?
The chart tracks influenza-like illness (ILI) — the percentage of doctor visits for fever plus cough or sore throat, the telltale signs of flu. It's the CDC's main real-time gauge of flu activity, reported weekly. ILI doesn't confirm flu in a lab, but it closely tracks the real thing and captures the season's shape and severity. A national baseline of around 2% marks the line above which flu is considered actively circulating.
The annual flu season
The chart's most obvious feature is its regularity: a sharp spike every winter, a flat trough every summer, repeating year after year. Flu thrives in cold, dry, indoor conditions, so activity reliably climbs from late fall, peaks somewhere between December and February, and collapses by spring. The height of each spike varies — a bad season peaks well above a mild one — but the timing is remarkably consistent, which is why flu shots are pushed every autumn.
The winter flu vanished (2020–21)
Look at the 2020–21 winter and you'll see something extraordinary: the usual spike almost completely disappeared. The same masking, distancing, school closures, and reduced travel that were deployed against COVID-19 also crushed the flu, which spreads the same way. It was one of the mildest flu seasons ever recorded — a real-world demonstration that those measures work against respiratory viruses. Flu came roaring back in later seasons as life returned to normal.
How bad can a flu season get?
In a severe season, ILI can climb several times above baseline, and flu sends hundreds of thousands of Americans to the hospital and causes tens of thousands of deaths, concentrated among the elderly and very young. Severity depends on which strains dominate and how well that year's vaccine matches them. The chart's tallest spikes mark the worst seasons — a reminder that flu, though routine, remains a serious annual threat, not just an inconvenience.
Frequently asked questions
What is influenza-like illness (ILI)?
The share of doctor visits for fever plus cough or sore throat — the CDC's main weekly gauge of flu activity. A national baseline near 2% marks active flu circulation.
When is flu season in the U.S.?
Flu activity climbs from late fall, peaks between December and February, and fades by spring — a remarkably consistent annual pattern driven by cold, dry, indoor conditions.
Why did the flu nearly disappear in 2020–21?
The masking, distancing, school closures, and reduced travel used against COVID-19 also stopped the flu, which spreads the same way, producing one of the mildest flu seasons on record.
How bad can a flu season be?
In severe seasons, flu-like illness climbs several times above baseline, hospitalizes hundreds of thousands, and kills tens of thousands — mostly the elderly and very young.
What makes one flu season worse than another?
Mainly which flu strains dominate and how well that year's vaccine matches them. A poorly-matched vaccine or a harsh strain produces a more severe season.