What's going around in the United States?

The CDC does not publish a single national “how many are sick” figure, but weekly hospital admission rates for flu, COVID-19, and RSV together give a strong proxy. When all three are active at once, as often happens between December and February, the combined respiratory-illness burden can strain hospital capacity. On infectrisk.com we classify the overall picture as low, moderate, or high based on these hospitalization signals. For exact figures and personalized risk, the InfectRisk app provides deeper analysis.

“Tripledemic” is the media shorthand used during the 2022–2023 winter for the simultaneous peak of influenza, COVID-19, and RSV: the first season in which all three were circulating at high levels at the same time post-pandemic. Since then, the three viruses regularly co-circulate during the US winter respiratory season, though the intensity of any given season varies.

Several factors drive the winter peak. Many respiratory viruses survive longer in cold, dry indoor air, and heated buildings create the dry conditions that impair the protective function of airway mucous membranes. People spend more time indoors in close contact, and lower humidity favors aerosol transmission. On top of that, the start of school and holiday gatherings mix populations more intensely during the colder months.

“Immune debt” refers to the idea that, after periods of low pathogen circulation, for instance during COVID-19 lockdowns, population-level immunity to certain viruses declines. Subsequent waves can then hit harder because regular immune “training” was missing. This concept partly explains the unusually severe RSV and flu seasons seen in 2022–2023. It is not a permanent immune defect, just a temporary catch-up.

The CDC uses historical baselines and defined thresholds. For influenza-like illness, the CDC compares current outpatient visits against the average for past non-epidemic weeks. For hospitalizations, threshold crossings into the high range reflect weekly admission rates above past severe-season reference points. These data-driven classifications help distinguish unusual surges from normal seasonal activity.