San Francisco school TB outbreak expands, 18% of those tested infected

At Archbishop Riordan High School in Westwood Park, nearly 1 in 5 students and staff tested for tuberculosis had a positive result, turning a campus health concern into one of San Francisco’s largest recent school outbreak responses. By April 27, 1,404 people, or 96% of the school population, had at least one TB test reviewed and validated by the San Francisco Department of Public Health, and 252 people, 18%, had a positive test. Health officials said 7 confirmed active TB cases had been identified among those exposed.

The outbreak began in November 2025, when SFDPH said the first active case was diagnosed at Riordan. The second and third active cases were diagnosed in January 2026. By Feb. 20, the city said it had confirmed 3 active cases and 3 additional suspected active cases, after receiving 1,261 test results from students and staff. Of those, 219 people, or 17%, had a positive TB test, and 204 latent TB infections had been confirmed. SFDPH said no related active TB cases had been reported outside Riordan and that the general public risk remained low.

For families trying to sort out what the numbers mean, the key distinction is between latent and active TB. SFDPH’s January advisory said latent TB infection is not contagious, but it can become active if untreated. The health department also said repeat TB assessment would be required every 8 weeks until the outbreak was resolved, a sign that this was not a one-time screening event but an ongoing investigation that could stretch across the school year.

The outbreak also disrupted classes. Riordan canceled in-person instruction in late January, then brought students back to campus on Feb. 24. SFDPH and the school said people with suspected or confirmed active TB were not allowed on campus until medically cleared. The school said the last date an infectious TB case was on campus was Feb. 19.

By late April, Riordan said schoolwide testing had been completed in March, with another on-campus round planned for May 6 and May 8 for higher-risk groups. The school said 241 people had confirmed latent TB infection and that 95% of those diagnosed had started treatment or finished it. For San Francisco, the episode was a reminder that an old disease can still spread quietly through a close-knit community before its full scale becomes visible, even in a school with strong public-health support.