By the start of the 20th century, tuberculosis was one of the leading causes of death in the United States.

Governments in the early part of the last century began making serious progress in preventing infectious diseases. According to the CDC, housing improvements reduced overcrowding — a risk factor for TB spread — and programs were launched to control TB. By 1940, the TB death rate was one-fourth what it had been at the turn of the century. Further progress was made with the discovery of TB treatments in the middle of the 20th century.

TB cases have generally been declining in California since the early 1990s but continue to take a significant toll, a tragedy given the disease is detectable and treatable.

"Not only can you treat the active disease, you can also treat the latent disease, which is much easier to treat — fewer drugs, much shorter time — and then prevent that progression to the active form of disease," Higashi said.

The CDC estimates 13 million people in the U.S. have a latent tuberculosis infection — meaning they have no symptoms and can't spread the bacteria. Without treatment, 5% to 10% will develop active disease that can turn contagious and potentially deadly later in life.

Outbreaks can happen locally. In Contra Costa County last year, 11 confirmed cases were linked to staff and customers of the California Grand Casino in Pacheco. At least 10 cases were genetically linked.

According to the most recent data, California counties with the highest per capita rates of tuberculosis are generally along populous coastal areas. In Southern California, those with the highest rates were Los Angeles, San Diego, Orange and Imperial counties. In Northern California, they were Santa Clara, Alameda, Sacramento, Contra Costa, San Francisco, San Mateo, San Joaquin, Solano, Monterey and Napa counties.

Officials noticed a substantial drop in TB cases in 2020, tied to stay-at-home orders and reduced travel during the early phase of the pandemic. Since then, case rates have increased 4% to 5% each year before jumping by 15% in 2023.

The rise in TB cases in Orange County, California's third most populous, has been especially pronounced, jumping 20% between 2022 and 2023; in L.A. County, cases rose by about 4% over the same time, according to local data.

According to state data, Orange County's per capita TB case rate is 10% higher than L.A. County's.

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