Several viruses are circulating in California right now, with seasonal influenza, COVID-19, and RSV leading the way as the main respiratory threats. Measles is also making a notable appearance, with localized outbreaks in multiple counties. Here’s what’s going around, how each virus shows up differently, and what to watch for.
Influenza A Is the Dominant Flu Strain
The flu strain hitting California hardest this season is a variant of influenza A H3N2, specifically a newer subtype known as “subclade K.” H3N2 seasons tend to be rougher than H1N1 seasons, particularly for older adults and young children. Symptoms come on fast, often within one to two days of exposure: sudden fever, body aches, chills, fatigue, and a dry cough. If you suspect the flu, antiviral treatment works best when started within 48 hours of your first symptoms and can shorten illness by a day or two while reducing the risk of complications like pneumonia.
COVID-19 Is Still Circulating
COVID-19 hasn’t disappeared from the mix. It continues to circulate alongside flu and RSV, and its symptoms overlap heavily with both: cough, congestion, headache, fatigue, and sometimes fever. Loss of taste or smell, once a hallmark of earlier variants, is less common now but still occurs. The main practical difference is that antiviral treatment for COVID-19 has a wider window, working best when started within five to seven days of symptom onset. A home rapid test is the quickest way to tell COVID apart from the flu, since symptoms alone won’t reliably distinguish the two.
RSV Remains a Concern for Infants and Older Adults
Respiratory syncytial virus peaks in fall and winter alongside flu season. For most adults, RSV feels like a bad cold. For infants under a year old, older adults, and people with chronic lung or heart conditions, it can turn serious quickly, sometimes causing bronchiolitis or pneumonia.
The good news: newer prevention tools are making a measurable dent. RSV-associated hospitalization rates among infants under 8 months old dropped 43% during the 2024-25 season compared to pre-pandemic baselines, falling from about 15 per 1,000 children to 8.5 per 1,000. That decline is tied to the availability of preventive treatments given to newborns and vaccines offered during pregnancy. California is one of the states included in the CDC’s active surveillance network tracking these hospitalizations.
Norovirus Is Driving Stomach Bug Outbreaks
If the illness going around your household or workplace involves vomiting and diarrhea rather than coughing, norovirus is the likely culprit. California has seen elevated norovirus activity, with at least 25 confirmed outbreaks reported in the state during a recent surge. Those outbreaks hit long-term care facilities and at least one elementary school hard enough to require temporary closure.
Norovirus spreads incredibly easily through contaminated surfaces, food, and direct contact. Symptoms (nausea, vomiting, watery diarrhea, stomach cramps) typically appear 12 to 48 hours after exposure and last one to three days. Unlike respiratory viruses, alcohol-based hand sanitizer doesn’t kill norovirus effectively. Soap and water is what works. The virus is also notorious for surviving on surfaces for days, which is why it tears through households, schools, and cruise ships so efficiently.
Measles Outbreaks in Multiple Counties
California has confirmed 26 measles cases so far in 2026, with two active localized outbreaks. Cases have appeared across a wide geographic spread: Los Angeles, Orange, Riverside, Sacramento, San Mateo, Santa Clara, Napa, Placer, and Shasta counties. Fourteen cases had rash onset in January, with another 12 in February.
Measles is one of the most contagious viruses known. It lingers in the air for up to two hours after an infected person leaves a room. Early symptoms look deceptively like a cold (fever, runny nose, cough, red eyes), followed by the characteristic rash three to five days later. The virus is preventable with the MMR vaccine, which remains part of the recommended childhood schedule in California. If you’re unsure of your vaccination status and you live in or near an affected county, checking with your doctor is worth the effort.
Avian Flu: Limited to Specific Exposures
You may have seen headlines about bird flu in California. Between September and December 2024, 38 people in the state tested positive for highly pathogenic avian influenza A (H5N1). Of those, 37 were dairy farm workers who had direct occupational contact with sick cows. One case was a child with no known animal exposure, identified through routine flu surveillance.
The critical detail: no human-to-human transmission of H5N1 has been identified in the United States. The risk to the general public remains very low. This is primarily an occupational hazard for people working closely with infected poultry or dairy cattle and their raw milk. Pasteurized dairy products are not a concern.
How to Tell These Viruses Apart
The honest answer is that you often can’t tell from symptoms alone. Flu, COVID-19, and RSV all cause cough, congestion, headache, and fatigue. A few patterns can help narrow things down, though:
- Sudden onset with severe body aches leans toward influenza.
- Vomiting and diarrhea without much respiratory involvement points to norovirus.
- Wheezing in an infant raises suspicion for RSV.
- Red eyes followed by a spreading rash suggests measles, especially if you’re unvaccinated.
Combination home tests for flu and COVID are widely available at pharmacies and can give you a useful answer within 15 minutes. Knowing which virus you have matters because the treatment windows differ: 48 hours for flu antivirals versus five to seven days for COVID antivirals.
Vaccines Available for This Season
California’s Department of Public Health recommends flu vaccination for everyone 6 months and older. COVID-19 vaccines are recommended for young children (6 to 23 months), adults 65 and older, and anyone with conditions that raise the risk of severe illness. Adults who simply want protection can also get vaccinated.
RSV prevention follows a more targeted approach. For infants, protection comes either through a vaccine given during pregnancy (at 32 to 36 weeks) or a preventive dose given directly to newborns under 8 months. For older adults, RSV vaccination is recommended for those 75 and older, or starting at 50 with certain risk factors. Unlike flu and COVID shots, RSV vaccination is currently a one-time recommendation rather than an annual one.