California Sees Record-High Contagious Disease Outbreaks

Tuberculosis is the disease behind this story — not measles, not COVID, and not some vague “contagious illness.” California is dealing with a real TB rebound, and the most visible flashpoint is Archbishop Riordan High School in San Francisco, where a school outbreak turned into a statewide warning sign. The reason this matters is simple: TB still kills, it spreads through the air, and it exploits weak spots in public health systems. What changed this spring is that the numbers got concrete — both at the school and in new federal data showing how modern U.S. outbreaks actually spread. ### Why is this a TB story? TB is a bacterial disease that usually attacks the lungs. It spreads when someone with active lung TB coughs infectious particles into the air. But the tricky part is that many people carry latent TB infection — meaning the bacteria are in the body, but the person is not sick and cannot spread it. That split between active and latent infection is why outbreaks can look confusing at first. (media.api.sf.gov) ### What happened at Riordan? San Francisco public health officials first flagged the Riordan outbreak in January 2026. By March 4, the city said four active cases had been lab-confirmed, with three more suspected, and 227 of 1,359 tested students and staff had positive TB tests. Later reporting pegged the outbreak at 252 total positives tied to the school — 7 active cases and 241 latent infections — through late April. That is why the outbreak grabbed attention: not because everyone was contagious, but because the exposure footprint was huge. (cdph.ca.gov) ### Why does “latent” still matter? Latent TB is not contagious. But it is the reservoir that can turn into future disease if people do not get preventive treatment. San Francisco’s March update said 207 people at the school already had confirmed latent infection, and the whole point of follow-up screening was to stop those infections from becoming active later. Basically, the outbreak is not just about who got sick now — it is about how many future cases can still be prevented. (media.api.sf.gov) ### Is California really at a record high? The “record-high outbreaks” framing is a little slippery. California’s official statewide snapshot shows 2,109 people had active TB disease in 2024, almost unchanged from 2,114 in 2023, and the state says annual case counts have mostly stayed between 2,000 and 2,200 since 2012, aside from the pandemic dip. So this is not a single all-time statewide case record. The sharper point is that outbreaks are getting more visible again, and California’s burden remains high — with incidence nearly double the national rate. (media.api.sf.gov) ### What is the new national clue? The CDC’s new outbreak analysis is the part that changes the frame. Looking at 2017 through 2023, it found 50 large TB outbreaks in 23 states involving 1,092 cases. People in those outbreaks were much more likely to be U.S.-born — 79% versus 26% among other TB cases — and outbreaks were often tied to family or social networks, plus higher rates of substance use, homelessness, and incarceration. That undercuts the old reflex that TB outbreaks are mostly an imported problem. (cdph.ca.gov) ### So who carries most of California’s TB burden? California still has a very different baseline picture from outbreak clusters. In 2024, the state said people born outside the U.S. bore the largest overall burden of TB, with a rate 14 times that of U.S.-born residents. Most California TB also came from latent infection progressing into disease, not from recent transmission. So both things are true at once — the overall burden is still concentrated among immigrants, but large outbreaks increasingly look domestic and network-driven. (cdc.gov) ### Why does this stress clinics and health departments? TB control is slow, labor-heavy work. You need testing, chest X-rays, contact tracing, repeat screening, treatment monitoring, and often months of follow-up. Riordan alone required mass school testing and repeat evaluation for all students and staff. California’s health departments are not just treating sick patients here — they are managing a long chain of prevention work that only succeeds if people keep showing up. (cdph.ca.gov) ### Bottom line The real story is not that California suddenly discovered TB. It is that a disease many people think of as old and rare is still very good at finding gaps — and the newest evidence says those gaps are increasingly inside U.S.-born communities, not only at the border. (cdc.gov) (media.api.sf.gov)