Active Public Health Outbreaks in California: Deep Research Report
Current Status (November 30, 2025): California is currently managing a complex landscape of public health threats. While the high-profile H5N1 Bird Flu outbreak among dairy workers has stabilized with no new human cases since January, the state is facing a critical, active emergency regarding Infant Botulism linked to contaminated formula. Simultaneously, vaccine-preventable diseases like Measles and Pertussis are seeing significant resurgences, exceeding 2024 levels.
1. Critical Outbreak: Infant Botulism
As of late November 2025, the most urgent active outbreak involves a multi-state spike in Infant Botulism linked to powdered infant formula produced by ByHeart. The California Department of Public Health (CDPH) and CDC have identified a cluster of cases occurring between August and November 2025.
- Scope: Nationwide spike exceeding 100 cases (107 total).
- California Impact: Multiple confirmed cases; CDPH issued a specific health advisory on November 11, 2025.
- Timeline: Cases linked to this outbreak began appearing as early as November 2024, but a significant "spike" was detected from August 1 to November 19, 2025.
- Status: Active investigation. This is the first botulism outbreak associated with powdered infant formula in the U.S.
2. H5N1 Bird Flu: 2024-2025 Assessment
California was the epicenter of human H5N1 infections in late 2024, driven by outbreaks in dairy cattle. However, human transmission has stalled in 2025.
Latest Data (Nov 30, 2025):
The cumulative count for California stands at 38 confirmed human cases. Crucially, the last confirmed human case in the state was reported on January 14, 2025. Since then, despite ongoing surveillance of thousands of exposed workers, no new human infections have been detected in the state through November 2025.
H5N1 Case Breakdown
3. Resurgence of Vaccine-Preventable Diseases
| Disease | 2025 Cases (YTD) | Comparison (2024) | Status |
|---|---|---|---|
| Measles | 22 (as of Nov 24) | 15 Total | Rising |
| Pertussis (Whooping Cough) | ~1,915 (Jan-Sep) | 1,917 (Jan-Sep) | Sustained High |
| Valley Fever | >5,500 (Provisional) | 12,500 (Record Year) | Elevated |
Measles & Pertussis
Measles activity has exceeded 2024 totals, with 22 confirmed cases across 13 jurisdictions including Los Angeles, San Diego, and Sacramento. Pertussis remains at pre-pandemic levels, with nearly 2,000 cases reported in the first three quarters of 2025. Health officials note a "cyclical increase" exacerbated by waning immunity and vaccination gaps.
Valley Fever
Following a record-breaking year in 2024 (12,500 cases), Valley Fever continues to surge. Provisional data indicates over 5,500 cases in the first half of 2025 alone. The disease is expanding geographically beyond the Central Valley into coastal regions like Monterey and Santa Barbara.
4. Emerging & Vector-Borne Threats
Locally Acquired Dengue (2025)
Status: Local transmission recurred in 2025, specifically in the San Gabriel Valley (LA County), confirming that Aedes mosquitoes are successfully vectoring the virus within the state.
Candida auris: While a specific outbreak in Alameda County was declared over in August 2025, "ongoing transmission" continues in healthcare facilities across Northern and Central California. The state has recorded over 10,000 cumulative cases since 2017.
Silicosis: An occupational health emergency has been declared regarding silicosis among countertop workers. As of November 13, 2025, there are 432 confirmed cases, prompting the state to make it a reportable disease to better track this preventable but fatal lung condition.