Communicable Diseases Quarterly Report Q3 2026 Posted on November 9, 2026 Quarterly update on communicable disease trends and case counts in San Mateo County. Issue No. 65 | Data through: September 30, 2026 Catherine Sallenave, MD, CD Controller | Kismet Baldwin-Santana, MD, Health Officer Focus on: Complications of Measles — Part I According to Centers for Disease Control and Prevention data, 2,289 confirmed measles cases were reported in the United States during 2025. As of November 5, 2026, 5,182 confirmed cases had been reported nationally in 2026. Forty-three new outbreaks were reported in 2026, and 96% of confirmed cases—4,993 of 5,182—were outbreak-associated. State health department reporting may exceed federal totals. For example, while the federal count listed four measles deaths, the Pennsylvania Department of Health had reported six confirmed measles deaths, with additional deaths under review. Although measles is commonly recognized by its maculopapular descending rash and Koplik spots, complications occur in approximately 30% of cases. Koplik spots may appear inside the mouth 24 to 48 hours before the skin rash. Gastrointestinal complications. Diarrhea is the most common complication, occurring in approximately 8% of cases. Other gastrointestinal complications include gingivostomatitis, gastroenteritis, hepatitis, mesenteric lymphadenitis, and appendicitis. Pulmonary, cardiac, and ocular complications. Pneumonia, which occurs in approximately 6% of cases, is the most common cause of measles-associated death in children. Other pulmonary complications include bronchopneumonia, croup, bronchiolitis, and bronchiectasis. Bacterial superinfection may occur in up to 5% of cases. Cardiac complications include myocarditis and pericarditis; ocular complications include keratitis and corneal ulceration. Neurologic complications. Measles can cause encephalitis, acute disseminated encephalomyelitis (ADEM), and subacute sclerosing panencephalitis (SSPE). Encephalitis occurs in up to 1 per 1,000 measles cases and usually appears within days of rash onset. Symptoms may include fever, headache, vomiting, stiff neck, meningeal irritation, drowsiness, convulsions, and coma. Acute measles encephalitis may occur without rash. Cerebrospinal fluid findings in measles encephalitis may include predominantly lymphocytic pleocytosis, elevated protein concentration, and normal glucose concentration. Approximately 25% of affected children have neurodevelopmental sequelae, and rapidly progressive fatal disease occurs in approximately 15% of cases. ADEM is a demyelinating disease that occurs in approximately 1 per 1,000 measles cases. It is believed to be a postinfectious autoimmune response and typically occurs during recovery, within two weeks of rash onset. Measles-associated ADEM has a reported mortality of 10% to 20%; residual neurologic abnormalities among survivors may include behavioral disorders, cognitive deficits, and epilepsy. Selected Communicable Disease Cases Reported in San Mateo County Disease 2026 2025 3rd Qtr YTD 3rd Qtr YTD Coccidioidomycosis* 10 35 13 46 Dengue 6 7 5 9 Legionellosis* 4 11 4 5 Malaria* 0 0 1 2 Meningitis/Encephalitis* 3 15 6 16 Bacterial† 2 4 3 6 Fungal‡ 1 1 1 3 Unknown 0 1 0 1 Viral§ 0 9 2 6 *Includes confirmed cases only. †Excluding meningococcal meningitis. ‡Excluding coccidioidomycosis. §Excluding West Nile virus. Selected Gastrointestinal Illnesses Reported in San Mateo County Disease 2026 2025 3rd Qtr YTD 3rd Qtr YTD Campylobacteriosis 146 333 134 335 Cryptosporidiosis 8 23 12 24 Cyclosporiasis 13 17 9 9 Giardiasis 22 64 28 83 Listeriosis 2 3 0 0 Paratyphoid fever 0 0 1 1 Salmonellosis, non-typhoidal 57 129 53 133 Shigellosis 38 92 20 64 Group A (dysenteriae) 0 1 0 0 Group B (flexneri) 1 12 1 9 Group C (boydii) 0 0 0 1 Group D (sonnei) 4 14 5 9 Unspecified 33 65 14 45 STEC* with HUS 0 0 1 1 STEC* without HUS 45 102 38 97 Typhoid fever 0 1 0 0 Vibriosis, non-cholera 8 12 9 10 Yersiniosis 19 31 5 26 *Shiga toxin-producing Escherichia coli. Selected Vaccine-Preventable Diseases Reported in San Mateo County Disease 2026 2025 3rd Qtr YTD 3rd Qtr YTD Hepatitis A* 1 1 1 1 Hepatitis E** 1 1 0 2 Measles† 0 2 0 1 Pertussis† 7 44 11 50 *Includes confirmed cases only. **Hepatitis E is included in this section for surveillance purposes; Although a Hepatitis E vaccine is available in some countries, no Hepatitis E vaccine is currently licensed for use in the United States. †Includes confirmed, probable, and suspect cases. About the Communicable Disease Control Program The Communicable Disease Control Program is available to help meet the reporting needs and answer questions from San Mateo County providers. Provider reporting or report a disease/outbreak, call 650-573-2346 Monday through Friday, 8:00 a.m. to 5:00 p.m., or fax a Confidential Morbidity Report (CMR) to 650-573-2919. Download an electronic CMR at smchealth.org/communicablediseasereporting. Web-based reporting through CalREDIE is also available. Contact the program to learn more about and enroll in web-based reporting. Non-urgent questions and general inquiries: SMCCDControl@smcgov.org Data: California Reportable Disease Information Exchange (CalREDIE); data pulled October 21, 2026. Notes: For individual diseases, morbidity is based on the date the case was received by the CD Control Program. Totals for past quarters may change because of delayed reporting from laboratories and providers, use of different reporting systems, and changes to case resolution statuses based on subsequently received information. All totals are for confirmed and probable cases unless otherwise noted. Authors: Communicable Disease Control Program.