TB outbreak at SF high school affects nearly 1 in 5, further testing underway

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SAN FRANCISCO, Calif. - Nearly one in five students and staff at San Francisco’s Archbishop Riordan High School have been diagnosed with latent or active tuberculosis, the latest development in an outbreak that started in November.

On Wednesday, the school started a new round of testing, which will be completed on Friday, according to an April 27 letter from the school and the San Francisco Department of Public Health. Testing will be handled by SFDPH and Kaiser, respectively.

Testing will focus on students, faculty and staff who have been exposed to a recently confirmed case, or who are part of a small group of individuals who had previously tested positive for latent tuberculosis, a dormant form of TB.

Falling numbers

What they're saying:

The last round of tests, conducted in March, showed a "strong reduction" in transmission.

"Since March 9th, there have been 1,135 people who have new TB test results reported, of which 19 (2%) were positive. For the complete testing period, from (November) through (April 22), 1404 (96%) of the total school population have had at least one TB test reviewed and validated by SFDPH," the letter states. "A total of 252 (18%) had a positive test."

At the time the letter was sent, there were seven confirmed cases of active tuberculosis among those tested, 241 latent cases, and four cases waiting on X-ray results.

Individuals with latent tuberculosis are not contagious but the infection, if left untreated, could develop into active tuberculosis.

Those individuals diagnosed with a latent tuberculosis infection, as of April 27, were being treated or had completed treatment.

SF school cancels classes over TB outbreak

Archbishop Riordan High School is temporarily closing its doors and switching to hybrid learning, amid concerns about a tuberculosis outbreak. 

November surprise

The backstory:

The school has a population of around 1,100 students. Students and faculty, after the first case was detected in November, 2025, were required to test for tuberculosis between January20 and February 13. The infection’s incubation period can take up to 10 weeks to incubate.

The school’s president, Tim Reardon, shortly after the first outbreak, sent a statement to KTVU, saying in part, "We will take every measure available to ensure the safety and wellbeing of faculty, staff, students, and their families, and we are committed to providing frequent, consistent updates to members of the Riordan community."

TB presents like a cold or flu, and spreads easily through coughs and sneezes, so medical experts say it can be tough to detect.

Tuberculosis only becomes active in about 5-10% of people, but presents a higher risk for babies and the immunocompromised.

The disease can be treated with antibiotics.