NEW YORK — The number of U.S. tuberculosis cases in 2023 were the highest in a decade, according to a new government report.

Forty states reported an increase in TB, and rates were up among all age groups, the Centers for Disease Control and Prevention said Thursday. More than 9,600 cases were reported, a 16% increase from 2022 and the highest since 2013.

Cases declined sharply at the beginning of the COVID-19 pandemic, but have been rising since.

Most U.S. TB cases are diagnosed in people born in other countries. Experts say the 2023 number is in part a combination of a surge in TB cases internationally — the World Health Organization said TB was behind only COVID-19 in infectious fatal diseases worldwide in 2022. And there are also increases in migration and post-pandemic international travel.

But other factors are also at play, including other illnesses that weaken the immune system and allow latent TB infections to emerge.

CDC officials expected TB numbers would rise, but the 2023 count “was a little more than was expected,” said Dr. Philip LoBue, director of the agency's Division of Tuberculosis Elimination.

Despite the jump, the number and rate of new TB cases each year remains smaller than it was in the past, and the U.S. has a lower rate of new TB cases than most countries.

Tuberculosis is caused by bacteria that usually attack the lungs, and is spread through the air when an infectious person coughs or sneezes. If not treated properly, it can be fatal. In the late 1800s, TB killed one out of every seven people living in the United States and Europe. But the development of antibiotics and public health efforts succeeded in treating infections and tracking down those they infected, leading to cases falling for decades.

The new CDC statistics are not a count of how many people were newly infected in 2023, but rather of how many people developed a cough or other symptoms and were diagnosed.

An estimated 85% of the people counted in 2023 were infected at least a year or two earlier and had what’s called latent TB, when the bacteria enters the body and hibernates in the lungs or other parts of the body. Experts estimate as many as 13 million Americans have latent TB and are not contagious.

When the immune system is weakened — by certain medications or other illnesses like diabetes and HIV — the TB wakes up, so to speak. Nicole Skaggs said she was infected in 2020, but didn’t develop symptoms until 2022 — after she got sick from COVID-19.

“Anything that can take out or lower your immune system can put you at risk,” said Skaggs, 41, a property manager in Bothell, Washington.

CDC officials called the idea that COVID-19 has played a role in increased reactivation of TB “an important question." Scientists are still learning what causes latent TB to reactivate and "I would consider it an unknown at this point,” LoBue said.

“It’s too early to tell” what will happen to TB trends in the next few years, he also said.

There are TB vaccines being developed, and public health workers that were focused on COVID are now back to trying new approaches to preventing TB. New York City, which saw cases jump 28% last year, is hiring TB case managers and community health workers and increasingly using video monitoring of patients taking medications to keep treatment rates high, said Dr. Ashwin Vasan, the city’s health commissioner.

On the other hand, federal TB funding for state and local health department efforts has been flat, and one of the key antibiotics used against TB has been in short supply in recent years. Plus, drug-resistant TB infections have popped up in a fraction of cases.