No aspect of the bitter and enduring debate about Lyme disease—which I addressed in the magazine a few weeks ago—has been more contentious than the dispute over how many people actually become infected each year. Last year, the Centers for Disease Control and Prevention reported that thirty thousand Americans were diagnosed with Lyme disease, the most commonly reported tick-borne illness in the United States. Today, it bumped the number a bit—to “around” three hundred thousand.

The shocking difference is not as bizarre as it might seem. The C.D.C. has always said that the lower number represented solely those diagnoses confirmed by laboratories or reported by physicians; officials acknowledged that the real case load was higher. But this new number is pretty damn high, and it includes just those who have been diagnosed with the disease.

If caught early, Lyme is easily treated with antibiotics. But activists, and many researchers, have long contended that tens of thousands of people remain unaware that they have been infected—sometimes for years, during which the bacterium can spread to the heart, nervous system, and brain.

The new data, which was released last night at the 2013 International Conference on Lyme Borreliosis and Other Tick-Borne Illnesses, was based on three studies, all of which continue: one analyzes medical-claims data submitted to insurance companies from twenty-two million people, the second is a survey of clinical laboratories, and the last is a more general assessment of people who believe they may be infected by Lyme.

The numbers matter: underreporting of Lyme disease obscures the true burden of the illnesses, on individuals as well as on health-care systems. It also makes it harder to convince Congress to fund research. These numbers ought to scare people, even in Washington. Paul Mead, the chief of epidemiology and surveillance activity for the C.D.C.’s Bacterial Diseases Branch, put it bluntly: “This new preliminary estimate confirms that Lyme disease is a tremendous public-health problem in the United States.”

Many scientists have known this for years; now it’s time for everyone else to start paying attention.

Photograph by Kenji Aoki