<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/Flu11.30.19.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/Flu11.30.19.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/Flu11.30.19.jpg?v=ap&w=980&h=551 800w" > The latest map showing influenza-like illnesses in the U.S. as of Nov. 30, 2019. The red and orange areas indicate high levels of flu activity. (U.S. Centers for Disease Control and Prevention)

At a Glance The flu has already killed as many as 2,400 people this season in the U.S.

Twelve states reported high levels of flu activity in the most recent CDC report.

The CDC says as many as 2.5 million have been infected.

Experts say it's too early to tell if the rapid start means a more severe flu season is ahead. As many as 2,400 people have died from the flu in the United States since Oct. 1, according to the latest numbers from the Centers for Disease Control and Prevention.

Up to 2.5 million people have been infected and 29,000 hospitalized.

This year marks the earliest start to widespread flu outbreaks in more than 15 years.

(MORE: How to Dodge the Flu This Year)

While flu season doesn't have a set start and end date, health officials consider it underway when a significant number of doctor's office visits are related to flu-like illnesses for three weeks in a row, according to the Associated Press. That threshold has now been met.

Flu cases have risen steadily over the past few weeks and continue to increase.

Twelve states and Puerto Rico reported high levels of flu activity in the CDC's weekly report released Dec. 6. That was up from seven states the previous week. Fourteen states, twice as many as the week before, reported moderate flu activity.

"Last year marked the longest flu season in a decade, and now we are seeing this year’s flu season off to an alarmingly fast start ," U.S. Rep. Diana DeGette (D-Colorado) said during a hearing with health officials this week on the flu, the Boston Herald reported.

As many as 61,000 people died from the flu during the 2018-19 season.

While some experts think the early and rapid start to this year's flu season could mean a sign of a particular intense flu battle this year, others say it's too early to tell.

"There’s not a predictable trend as far as if it’s early it’s going to be more severe, or later, less severe," Scott Epperson, who tracks flu-like illnesses for the CDC, told the AP.

Dave Osthus, a statistician in flu forecasting at the Los Alamos National Laboratory, said the early start could indicate that a lot of people will be sick at the same time.

"This could be a precursor to something pretty bad," Osthus said. "But we don’t know."

The CDC says the best way to avoid the flu is to get vaccinated. Flu shots are recommended for anyone six months and older. Other preventive measures include frequent handwashing and avoiding others who are sick.

Symptoms of the flu include fever, headache, cough, sore threat, stuffy nose, muscle or body aches and fatigue. Adults 65 years and older, pregnant women and young children are among the groups most at risk .

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