California Sees Record-High Contagious Disease Outbreaks

Tuberculosis is the disease behind those “record-high outbreak” headlines in California. And the news is really two stories at once. One is statewide — California logged 2,150 TB cases in 2025, the most since 2013. The other is local and more vivid — San Francisco health officials are still managing a major outbreak tied to Archbishop Riordan High School, where hundreds of students and staff have tested positive for either active or latent infection. ### Wait — is TB really that contagious? TB spreads through the air, but not as easily as measles or COVID in its most explosive phases. Usually, people have to spend a lot of time sharing indoor air with someone who has active TB in the lungs or throat. The catch is that once TB gets into a close-knit setting — a household, social network, shelter, or school community — it can keep moving quietly for months before anyone realizes what’s happening. (cdph.ca.gov) ### What changed in California? The statewide number is the big shift. California reported 2,150 people with TB disease in 2025, up from 2,109 in 2024, with an incidence rate of 5.4 per 100,000 people. That is well above the national provisional rate of 3.0 per 100,000. In plain English — California is not just seeing more TB, it is carrying a much heavier burden than the country as a whole. (cdph.ca.gov) ### Why are people talking about native-born residents? Because one of the older assumptions about TB is getting shakier. TB in California has long been concentrated among people born outside the U.S., and that is still true overall. But recent state reporting and local coverage point to a growing share among U.S.-born residents, especially in outbreak settings connected to family and social networks, substance use, and other conditions that make spread and delayed diagnosis more likely. (cdph.ca.gov) ### What’s happening at the San Francisco school? Archbishop Riordan High School has become the clearest example of how a TB outbreak can snowball. San Francisco’s health department said the outbreak began in November 2025. By late April, 252 people connected to the school had tested positive for either latent or active TB. Seven were active cases, and 241 were latent infections. New rounds of testing have been underway because TB can take time to show up on a test after exposure. (sfgate.com) ### Latent TB sounds scary — what does it mean? Latent TB means the bacteria are in the body, but the person is not sick and is not contagious. Active TB means the infection is causing disease and can spread if it involves the lungs or throat. That distinction matters a lot here, because a headline about “hundreds infected” can sound like hundreds of contagious cases. That is not what officials are describing at the school. But latent cases still matter, because they can turn into active disease later if they are not treated. (sf.gov) ### Why is California vulnerable? Partly because TB thrives in the exact places where public health has the hardest job — crowded housing, repeated close contact, delayed medical care, and long chains of exposure that are hard to map. California also starts from a higher baseline than most states. Once you already have a larger pool of cases, it is easier for clusters to keep appearing. (cdph.ca.gov) ### So what’s the bottom line? This is not a statewide emergency in the sense of a fast-moving new virus. It is an old disease regaining ground. California’s 2025 TB total is the highest in 12 years, and the San Francisco school outbreak shows how quickly TB can spread when it gets missed early. The reassuring part is that TB is treatable. The uncomfortable part is that controlling it takes time, testing, and follow-through — exactly the things outbreaks tend to expose as weak spots. (cdph.ca.gov)