For the first time in more than two decades, the rate of newly diagnosed tuberculosis cases is not dropping, U.S. health officials said Thursday.

Indeed, the number of new cases actually rose a little last year — the first of increase since 1992.

Officials said they're not sure why it's leveled off.

"It's always concerning when we see progress stall — especially when there are proven interventions to prevent a disease," said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, in a statement.

"We will need new and expanded efforts" to drive TB rates down again, said Dr. Jonathan Mermin, another CDC official, in an interview.

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, TB can be fatal.

It once was a major cause of death and illness, and in the late 1800s 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.

Still, today as many as 13 million Americans have latent TB, meaning the bacteria live in their lungs but aren't causing any illness. People with latent TB aren't contagious.

Most new TB illnesses occur when the immune system is weakened in a person with latent TB — by another illness, by medications, or some other cause.

TB illnesses flared in the 1980s and early 1990s, largely because of the AIDS epidemic. But they have been falling since.

Last year, 9,563 new TB illnesses were diagnosed, up from 9,406 the year before, according to preliminary CDC data released Thursday.

That translates to about three cases per 100,000 people. The rate was nearly three times higher 20 years ago, but it has been stalled at three since 2013.

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About two-thirds of new cases each year are diagnosed in foreign-born immigrants. But while diagnosis rates continue to decline in foreign-born people, they have leveled off in those born in the United States.

Health officials say they've had success stopping spread of TB from sick people to others in the United States, but more work needs to be done to fight TB globally. And in this country, there still is a steady welling up of illnesses from people with latent infections.