This Flu Map Shows How the Biggest Influenza Outbreak in Years Spread Across the U.S.

With months left to go in the 2018 flu season, the U.S. has already hit an unfortunate benchmark, as shown on the flu map below: For the first time in the Centers for Disease Control and Prevention’s 13 years of influenza monitoring, every state in the continental U.S. is seeing “widespread” virus activity.

The U.S. is experiencing such an active flu season that the CDC held a special briefing on the topic last week, explaining that there’s an uptick in both confirmed cases of the disease and hospitalizations related to it this year. The flu is so widespread, in fact, that the agency has declared it an epidemic, and urged those who have not been vaccinated to seek out the flu shot.

But how did this year’s flu season get so bad? Spread of the disease was bolstered by the circulation of H3N2, an influenza strain that’s particularly aggressive and was only successfully targeted by vaccines in about 30% of cases, according to the CDC. As more people become sick with this type of influenza, logically, more people pass it on — leading to a flu epidemic that’s steadily spreading throughout the country as flu season presses on.

You can see how the influenza virus spread in this 2018 flu map, which is based on CDC data: