Infectious Disease Epidemiology Brief
Edition date: March 24, 2026
Coverage window: Most items below were published or updated between March 14 and March 24, 2026. A few older items are included because they remain operationally important.
Executive Summary
California’s late RSV season continues to affect pediatric care. State and local health authorities have advised providers to continue RSV immunization for eligible infants and young children through April 30, 2026, reflecting a season that started later than usual and has remained elevated into March (Santa Clara County Health, CDPH / CAHAN).
Measles remains the most operationally important vaccine-preventable threat in California. Confirmed California cases reached 29 in 2026 as of mid-March, with active investigations and exposure follow-up involving the Sacramento region, Santa Clara County, and San Diego healthcare settings (CDPH Measles, County of San Diego Measles 2026).
Influenza is still producing more acute respiratory burden than COVID-19 in at least some California settings. In San Diego County, recent surveillance showed that emergency visits and hospitalizations remained higher for influenza than for COVID-19, while RSV also continued to contribute to pediatric and hospital burden (County of San Diego Respiratory Virus Surveillance Report).
A California-linked food safety issue warrants close attention. Federal investigators are examining a multistate Shiga toxin-producing E. coli outbreak linked to RAW FARM raw cheddar cheese, with illnesses concentrated in very young children and no recall yet announced at the time of reporting (CDC E. coli outbreak page, FDA outbreak investigation).
Antimicrobial resistance and infection prevention remain active operational concerns. California has issued an alert on ongoing transmission of extensively drug-resistant Shigella, and state materials have reinforced infection prevention expectations for skilled nursing facilities, including dedicated infection preventionist staffing (CDPH / CAHAN, CDPH SNF Infection Prevention Resources).
Nationally, measles activity has escalated sharply, increasing the risk of imported or travel-associated introductions into California and heightening the need for rapid recognition, isolation, and vaccination catch-up (CDC Measles Data and Research).
Preparedness implication: Over the next 2–4 weeks, California agencies and health systems should prioritize measles front-door triage, late-season RSV prevention for infants, influenza treatment readiness, raw-dairy foodborne messaging, and healthcare workforce exposure management (Santa Clara County Health, County of San Diego Measles 2026).
California: Key Developments
1) Respiratory viruses
What happened
California extended RSV prevention guidance through April 30 because the 2025–26 season began later than usual and remains elevated. Local messaging indicates RSV infections in young children are likely to continue into April (Santa Clara County Health, CDPH / CAHAN).
Who is affected
The greatest concern remains infants and young children, especially those eligible for monoclonal antibody protection. In San Diego County, RSV activity remained meaningful, but influenza was still the dominant respiratory burden. Recent week-10 surveillance reported emergency visits of 2.0% for influenza, 0.3% for RSV, and 0.1% for COVID-19. Weekly hospitalizations were reported as 82 for influenza, 57 for RSV, and 19 for COVID-19 (County of San Diego Respiratory Virus Surveillance Report).
What we know vs. don’t know
We know the RSV season is prolonged and that influenza remains actively circulating in California, while current county surveillance suggests COVID-19 activity is comparatively lower. What remains uncertain is whether RSV and influenza activity will decline uniformly across counties in the next several weeks. Recent county counts are also preliminary and may change with reporting delay (County of San Diego Respiratory Virus Surveillance Report).
Operational implications
Local health departments and health systems should maintain RSV immunization access for eligible infants and continue influenza testing and prompt antiviral treatment when clinically indicated. Interim vaccine effectiveness estimates support continued influenza vaccination, but layered prevention and early treatment remain important in high-risk settings (Santa Clara County Health, CDC FluView, Week 10).
2) Vaccine-preventable diseases
What happened
California’s measles count reached 29 confirmed cases in 2026 by March 16. Recent investigations include healthcare-associated and public exposure settings, including a March San Diego healthcare exposure involving an unvaccinated out-of-state adult later diagnosed with measles, as well as a Santa Clara County case associated with international travel (CDPH Measles, County of San Diego Measles 2026).
Who is affected
The Sacramento region remains especially important because multiple cases were described in Sacramento/Placer, including pediatric cases and at least one case with unclear exposure source, raising concern for missed community transmission or incomplete exposure ascertainment (CDPH Measles).
What we know vs. don’t know
We know healthcare exposure management is now central to California’s measles response because recent events have involved emergency departments and public-facing sites. We do not yet know whether additional linked cases will emerge from current investigations in San Diego or the Sacramento region (County of San Diego Measles 2026, CDPH Measles).
Operational implications
Healthcare systems should immediately review front-door screening, airborne isolation workflows, employee exposure assessment, and rapid notification protocols. The policy environment in California appears stable: recent materials indicate no change to state vaccine recommendations or school immunization expectations (CDPH Measles, CDPH Public Health for All / What’s New).
3) Foodborne / waterborne disease
What happened
Federal agencies are investigating a multistate STEC outbreak linked to RAW FARM-brand raw cheddar cheese, a California-linked product. Reported illnesses span California, Florida, and Texas, with hospitalizations concentrated among very young children. No deaths or hemolytic uremic syndrome had been reported at the time of the update, and no recall had yet been announced (CDC E. coli outbreak page, CDC media release, FDA outbreak investigation).
What we know vs. don’t know
The epidemiologic evidence points toward raw cheddar exposure, but contamination had not yet been confirmed in product testing. That means the investigation is strong enough to justify precautionary messaging, even though final laboratory source confirmation may still be evolving (FDA outbreak investigation).
Operational implications
Clinicians should ask about raw dairy exposure when evaluating pediatric bloody diarrhea or possible HUS. Local health departments may want to target communication to families with young children. Separate gastrointestinal outbreak activity in congregate settings also suggests ongoing seasonal enteric risk (FDA outbreak investigation, CDC E. coli outbreak page).
4) Vector-borne / zoonotic disease
Current picture
No single new California vector-borne alert dominated the most recent reporting window, but this should be viewed as a preparedness opportunity rather than a signal of low long-term risk. Ongoing mosquito and arbovirus readiness remains important as seasonal conditions shift (CDPH Vector-Borne Disease Section).
At the same time, animal H5 bird flu activity continues to require monitoring, even though current public risk remains characterized as low (CDC FluView, Week 10).
Operational implications
Counties should use this relative lull to refresh mosquito surveillance readiness, summer arbovirus messaging, and animal-to-human exposure protocols rather than wait for seasonal acceleration (CDPH Vector-Borne Disease Section).
5) Healthcare-associated infections & antimicrobial resistance
What happened
California issued a health alert in March on ongoing transmission of extensively drug-resistant Shigella. State materials also reinforced expectations that skilled nursing facilities maintain a full-time, dedicated infection preventionist position (CDPH / CAHAN, CDPH SNF Infection Prevention Resources).
Why it matters
These are both operational and workforce issues. XDR Shigella can spread rapidly in healthcare, congregate, and shelter-associated environments, while infection prevention staffing requirements directly affect readiness for respiratory, gastrointestinal, and multidrug-resistant organism events (CDPH / CAHAN, CDPH SNF Infection Prevention Resources).
Operational implications
Hospitals, SNFs, shelters, and county partners should revisit enteric isolation procedures, laboratory susceptibility communication, and workforce protection plans, especially where staff work across multiple facilities or high-contact units (CDPH / CAHAN).
United States and Global Developments
U.S. measles
National measles activity has escalated sharply, with 1,487 confirmed cases across 32 jurisdictions reported as of March 19 and most cases linked to outbreaks. For California, this increases the likelihood of imported or travel-associated introductions and raises the operational importance of rapid detection and isolation (CDC Measles Data and Research).
Wastewater measles surveillance
Wastewater detections have been noted as a potential early signal before clinical recognition. This is operationally relevant because it may provide advance warning where clinical case identification lags (CDC Measles Wastewater Surveillance).
U.S. influenza burden
National influenza burden remains substantial, with millions of illnesses, hundreds of thousands of hospitalizations, and tens of thousands of deaths estimated so far this season. Vaccination coverage remains below 50% in both adults and children, reinforcing the need for layered prevention and treatment readiness (CDC FluView, Week 10, CDC MMWR).
Avian influenza
No new human H5 infections were reported in the latest federal update, and there remains no evidence of person-to-person spread in the United States. Risk remains low for the general public, but occupational vigilance is still warranted (CDC FluView, Week 10).
WHO food safety alert
Multiple countries have initiated recalls related to cereulide toxin contamination linked to ARA oil used in infant formula and related products. Traceability and root-cause work remain ongoing (WHO Disease Outbreak News DON596).
Global arbovirus signal
WHO continues to report a high dengue burden in the Western Pacific Region. This matters for California because imported dengue risk will increase as travel rises and invasive Aedes mosquito season returns (WHO Western Pacific dengue situation update).
Watchlist: Next 2–4 Weeks
- Measles in California healthcare and school settings — recent events involve emergency department exposure and incomplete exposure histories, making secondary transmission and delayed recognition key concerns (County of San Diego Measles 2026).
- Late-season RSV in infants and young children — prevention guidance has been extended through April 30, indicating ongoing risk and the need to sustain immunization access (Santa Clara County Health).
- Raw-dairy associated STEC — the implicated cheese product remains under active investigation without closure or recall resolution (FDA outbreak investigation).
- XDR Shigella transmission — control depends heavily on rapid recognition, infection prevention, and strong communication across healthcare and congregate settings (CDPH / CAHAN).
- H5N1 at the animal-human interface — human risk remains low, but animal activity continues to require occupational awareness and exposure precautions (CDC FluView, Week 10).
- Imported dengue / Aedes-season preparedness — global dengue burden remains high, and California vector conditions will become more permissive as temperatures rise (WHO Western Pacific dengue situation update).
Data Notes & Caveats
Respiratory and outbreak surveillance remain subject to reporting lag, particularly in the most recent week. Some county-level respiratory figures are explicitly labeled preliminary. Case counts may change after laboratory confirmation, deduplication, or reclassification. For the raw-cheddar outbreak, epidemiologic linkage currently appears stronger than laboratory source confirmation (County of San Diego Respiratory Virus Surveillance Report, FDA outbreak investigation).
One-Minute Takeaways
- Recheck measles triage and airborne isolation workflows in emergency, urgent care, and pediatric settings (County of San Diego Measles 2026).
- Keep RSV immunization access open through April 30 for eligible infants and young children (Santa Clara County Health).
- Expect influenza, not COVID-19, to remain the more immediate respiratory operational burden in at least some California settings (County of San Diego Respiratory Virus Surveillance Report).
- Advise families with young children to avoid raw milk and raw cheddar while the STEC investigation continues (CDC E. coli outbreak page).
- Review infection prevention staffing and enteric precautions in SNFs, hospitals, shelters, and other congregate settings (CDPH SNF Infection Prevention Resources, CDPH / CAHAN).
- Consider host vulnerability, not only engineering controls, when assigning staff to high-risk TB care settings (CDC Emerging Infectious Diseases).
Optional Add-On: New Evidence
A recent Emerging Infectious Diseases report described occupational transmission of extensively drug-resistant tuberculosis to a healthcare worker receiving TNF-α inhibitor therapy despite airborne precautions. The practical implication is that staffing decisions for high-risk TB care settings should account for host vulnerability in addition to engineering and administrative controls (CDC Emerging Infectious Diseases).