Nearly 1 in 5 affected

'This is a big outbreak': Nearly 1 in 5 affected by TB at SF school

By Gillian Mohney

New data shows 18%, or nearly 1 in 5, of tested students and staff at Archbishop Riordan High School in San Francisco were diagnosed with either latent or active tuberculosis during an outbreak that started in November. 

In total, 96% of the school community was tested, seven people were diagnosed with active cases during the course of the outbreak and 241 latent cases were reported, according to data released by the San Francisco Department of Public Health on April 27 to the school community. 

In February, four active cases and three suspected active cases were reported. The latest numbers reflect the suspected cases were confirmed to be active TB. The entire school community went through another round of TB testing in March. 

“Data from the March 2026 round of testing indicate a strong reduction in transmission,” the department said in an emailed statement to SFGATE Wednesday.   

The department also stressed that “repeat mass testing at the school is no longer required.” They said the new testing is “out of an abundance of caution” and only for those who had a recent exposure to a confirmed case or were part of a “small group of individuals with new latent TB cases” found during the March testing. 

Dr. Monica Gandhi, an infectious disease expert at UC San Francisco, explained that TB is a slow-growing disease, which means testing may not start for eight to 10 weeks after an exposure to active TB. 

“Every time there’s an active case, you start the clock from there in terms of rounds of testing,” she told SFGATE in a phone call. “If that person was at school, then everyone around them needs contact tracing and testing again.”

Gandhi noted that the California Department of Public Health defines a TB cluster as being four or more active cases.

“This is a big outbreak,” she told SFGATE of the seven active cases reported.

Tuberculosis, an ancient bacterial disease that dates back 9,000 years, can be either active or latent, according to the U.S. Centers for Disease Control and Prevention. When it’s active, the disease can be transmitted and also cause symptoms like cough, fever and weight loss. In latent form, it won’t cause symptoms or be transmitted. However, the disease can turn into an active case at any time, even decades after the initial infection.

Both active and latent TB can be treated with an antibiotic treatment that typically takes months.

While latent TB doesn’t result in symptoms, Gandhi said it was unusual to see such a high percentage of the school population be diagnosed with the condition.

“Kids in this country do not have latent TB like that,” she said. “Those kind of numbers, of 20% having latent TB, are in low-income countries.” 

Requests for further comment from Archbishop Riordan High School were not answered by the time of publication.

The  California Department of Public Health noted that the last time an infectious person was on campus was Feb. 19. As of March 9, only students who had been tested for TB and “have TB clearance on file with the school” are allowed on campus or at off-campus school events. The department said 95% of those diagnosed at the school with latent TB are undergoing or have finished treatment, and all of those who had been diagnosed with active TB were being treated.

Tuberculosis remains a problem for California, where cases hit a 12-year high in 2025 with 2,150 reported. The state also had a “substantially higher” rate of disease in 2025 than the U.S. overall, the department said, with 5.4 infections per 100,000 people compared with about 3 per 100,000 nationwide.