“They are busy, and they are overbooked,” she said. “They have seen a lot of kids — and they all have the flu.”

For most people who catch it, influenza is unpleasant but not serious. But the disease can cause as many as 49,000 deaths a year, primarily among older people and those with compromised immune systems. The flu that’s circulating this year is a type called H3N2, which tends to spread widely and can cause serious illness.

Right now, the flu is spreading fastest in the Southeast and the Midwest. Dayton, Ohio, and Knoxville, Tenn., are some of the places experiencing intense flu seasons, according to the Google data. But because the flu tends to move around the country, places that have been spared so far may just end up with more sick people later on.

Doctors know more than ever about where the flu is and how fast it is spreading, but Dr. Jhung said the medical system still had not made much headway in reducing the toll of influenza. Though the C.D.C. now recommends vaccination for everyone older than 6 months, vaccinations remain at less than 50 percent of the population. With so few people protected, it’s still relatively easy for the virus to spread each year; there are lots of susceptible people, and the airborne virus moves around quickly.