That is, in terms of the percentage of outpatient and emergency department visits involving patients with influenza-like illness (ILI). The Centers for Disease Control and Prevention (CDC) reports data in calendar weeks. During week five of this year, the week ended Feb. 1, ILI patients accounted for 7.8% of outpatient visits—the highest percentage recorded among data going back to the 1997–98 flu season. Previously, activity had been highest (7.7%) the week ended Oct. 24, 2009, during the H1N1 swine flu pandemic.

“Seasonal influenza activity remains elevated and continues to increase across the country,” the CDC said in a Feb. 7 report.

ILI symptoms include fever and cough or sore throat, not necessarily representing laboratory-confirmed flu. Meaning, some patients included in ILI records may have another respiratory illness such as COVID or RSV. Even so, lab-confirmed flu tests were at a 31.6% positivity rate the week ended Feb. 1, a statistic the Center for Infectious Disease Research and Policy called “eye-popping.” Just four weeks earlier, the week ended Jan. 4, nationwide flu test positivity had been 18.2%.

Respiratory illness activity remained “very high” as of Feb. 7, the most severe level of the CDC’s five-tier ranking. The agency highlighted these trends in emergency department visits:

• Flu: Very high; increasing
• RSV: Moderate; decreasing
• COVID: Low; decreasing

Viral flu activity was also considered very high in U.S. wastewater samples as of Feb. 7. This is an important measure of flu spread, the CDC says, because people who have the flu but don’t have symptoms often shed the virus into sewage. And because not everyone who is symptomatic will get tested for the flu or seek medical care, wastewater samples offer another way of tracking infection.

This flu season alone, the CDC has reported at least 24 million illnesses, 310,000 hospitalizations, and 13,000 deaths. Nearly 60 of those deaths were among children and, by far, the largest proportion of ILI outpatient visits has been among children under 5. Last week, the Today show reported that schools and school districts in at least 10 states had closed owing to an overwhelming number of sick staff and students.

Read more from Fortune

Where is flu activity highest?

That flu activity is peaking now isn’t unexpected. You can catch the flu year-round, and peak season in the U.S. tends to be between December and February. February, though, is the clear winner. Since the 1982–83 season, flu activity has peaked in February 17 times, more than any other month.

Flu activity was high or very high in 45 states and territories the week ended Feb. 1. Within the very high category, activity was most intense in:

• Massachusetts
• Michigan
• Nebraska
• New Hampshire
• New Jersey
• New Mexico
• New York City
• Ohio
• South Carolina
• Tennessee
• Texas

Montana was the only state in the lower 48 with minimal activity.

What are the symptoms of the flu?

Because flu symptoms can be similar to those of other respiratory illnesses, taking an at-home rapid test or getting tested at a health care facility is the only way to be certain you have the virus. The CDC advises watching out for these symptoms:

• Cough
• Fatigue
• Fever or feeling feverish/chills (not everyone will have a fever)
• Headaches
• Muscle or body aches
• Runny or stuffy nose
• Sore throat
• Vomiting and diarrhea (more common in children)

It’s not too late to get your 2024–25 flu shot

Though this season’s flu shot has been widely available since early September, you can still protect yourself, your loved ones, and your community by getting your annual jab now. Keep in mind that it takes your body up to two weeks after immunization to develop immunity.

“If you’ve got virus circulating in your community, get the vaccine,” Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, told Fortune at the start of the season. “That may help reduce the severity of your infection, even if it doesn’t necessarily prevent infection because you got it a little late.”

About 44% of adults and 45% of children had gotten their flu shot as of the week ended Jan. 25.