Los Angeles County, home to roughly ten million people, deals with overlapping waves of respiratory and gastrointestinal illness throughout the year. The usual suspects include influenza, COVID-19, RSV, rhinovirus, and norovirus, but the region also faces less familiar threats like coccidioidomycosis (Valley Fever) and wildfire smoke that makes respiratory infections worse. What’s “going around” at any given time depends on the season, air quality, and which viral strains happen to be circulating, and LA’s size and connectivity mean it rarely deals with just one bug at a time.
The Respiratory Viruses That Hit LA Hardest
When people feel a cold or flu coming on in Los Angeles, it’s easy to assume it’s either influenza or COVID-19. Both circulate heavily during the fall and winter months, and both can cause fever, body aches, congestion, and fatigue. But multi-year surveillance data from an LA County academic health system shows that rhinovirus, the so-called “common cold” virus, is one of the most significant drivers of severe respiratory illness in the area, particularly among young children. Along with RSV and human metapneumovirus, rhinovirus was linked to the highest disease severity across study years, and rhinovirus-driven illness occurred independently of co-infection with other viruses, meaning it wasn’t just tagging along with something else.1Open Forum Infectious Diseases. Multi-year Analysis of Respiratory Viral Dynamics at a Los Angeles County Academic Health System Reveals Significance of Rhinovirus in Young Children with Severe Respiratory Illness
That finding matters because rhinovirus doesn’t have a vaccine, doesn’t respond to antivirals, and gets dismissed as “just a cold.” For most healthy adults, it is just a cold. But in young kids, older adults, and people with asthma or other lung conditions, it can lead to serious lower-respiratory illness. RSV follows a similar pattern: mild in healthy adults, dangerous in infants and the elderly. Human metapneumovirus, which got more public attention in early 2025, has been circulating in LA for years and also peaks in winter and early spring.
The practical takeaway is that the “sickness going around” is almost never a single virus. At any given point in fall or winter, LA residents may be dealing with influenza, COVID-19, RSV, rhinovirus, and metapneumovirus simultaneously. Each one has a slightly different peak season, different severity depending on your age, and different treatment options. During the 2020–2021 season in California, about 23% of people who tested positive for influenza were simultaneously co-infected with SARS-CoV-2, with influenza B predominant among those co-infected.2Emerging Infectious Diseases. Influenza and SARS-CoV-2 Co-infections in California, USA, September 2020–April 2021 Co-infections make it harder to know what you’re dealing with based on symptoms alone.
Stomach Bugs and Norovirus Outbreaks
Not every illness going around is respiratory. Norovirus, the leading cause of vomiting and diarrhea outbreaks, hits LA County regularly, especially in winter months. Norovirus is extraordinarily contagious: it takes only a tiny number of viral particles to cause infection, it spreads through contaminated food, water, surfaces, and person-to-person contact, and it can survive on surfaces for days.
A notable example of how fast norovirus moves through the region came in late 2023 and early 2024, when concurrent outbreaks tied to raw oyster consumption struck multiple jurisdictions including Los Angeles County. Roughly 400 people across eight local health jurisdictions in California reported gastrointestinal illness, and investigators identified 179 outbreak-associated cases with 24 laboratory-confirmed norovirus infections. Patient samples also turned up sapovirus, astrovirus, rotavirus, and adenovirus, meaning the oysters were carrying a cocktail of enteric viruses, not just one.3PubMed Central. Concurrent Norovirus Outbreaks Associated with Consumption of Oysters Harvested in Mexico – California, December 2023-January 2024
Food-borne outbreaks grab headlines, but most norovirus transmission in LA is mundane: a child picks it up at daycare, brings it home, and half the household is sick within 48 hours. The illness typically lasts one to three days and resolves on its own, but dehydration can be serious for young children and older adults. There’s no antiviral for norovirus, so treatment is fluids and rest. Handwashing with soap and water is more effective than alcohol-based sanitizer for this particular virus, since norovirus is a non-enveloped virus that alcohol doesn’t kill as reliably.
How LA Tracks What’s Circulating
One reason public health officials in Los Angeles can speak to what’s going around is the region’s investment in surveillance systems that go beyond traditional doctor-visit reporting. Two approaches have become especially important: community-based text message surveillance and wastewater monitoring.
A text-message-based system called AiA (short for “Abriendo Investigación en Acción”) tracked COVID-19 symptoms across LA County from 2020 through 2024 by collecting self-reported data directly from community members. When researchers compared AiA’s signals against three standard surveillance methods, the correlation with reported COVID-19 case counts was remarkably strong at 0.975, and the system also correlated well with wastewater SARS-CoV-2 concentrations (0.861) and sentinel lab test positivity (0.762). Interestingly, a moderate correlation also showed up between AiA data and influenza test positivity, suggesting that text-based symptom reporting can pick up non-COVID respiratory illness too.4PubMed Central. Community-Driven, Text Message-Based COVID-19 Surveillance System, Los Angeles County, California, USA, 2020-2024
Wastewater monitoring works differently. Instead of asking people how they feel, researchers sample sewage and test for viral genetic material. Samples collected at the Hyperion Water Reclamation Plant, which serves about four million LA residents, have been analyzed using deep sequencing that can detect not just SARS-CoV-2 but a wide range of human-infecting viruses.5PubMed Central. Deep metatranscriptomic sequencing data of wastewater from Los Angeles, USA, 2023-2024 Earlier wastewater studies across Southern California’s treatment plants, covering roughly 16 million people, identified coronaviruses, influenza A, and noroviruses in the water, along with specific SARS-CoV-2 variants and mutations that were circulating in the community.6PubMed Central. RNA Viromics of Southern California Wastewater and Detection of SARS-CoV-2 Single-Nucleotide Variants
For the average LA resident, the practical benefit is that public health agencies can detect surges and new variants before hospital data catches up. If you check the LA County Department of Public Health’s dashboards and see wastewater signals climbing for a particular virus, that’s a useful early warning that the bug is spreading even if case counts haven’t spiked yet.
Wildfire Smoke Makes Respiratory Illness Worse
Los Angeles has a complication that most other major metro areas don’t face on the same scale: seasonal wildfire smoke. Fires in and around Southern California send fine particulate matter (PM2.5) into the air, and a growing body of evidence ties that smoke exposure to higher rates of infectious disease.
A systematic review and meta-analysis found that for every 10 micrograms per cubic meter increase in short-term wildfire PM2.5 exposure, COVID-19 infections rose by about 15%. The same analysis found a roughly 3% increase in broader respiratory diseases and a 3% increase in acute upper respiratory infection combined with acute bronchitis at the same exposure increment. The effect wasn’t limited to viral infections: medium-term wildfire smoke exposure (more than a month but less than a year) was associated with a 20% rise in hospitalizations for systemic fungal infections like coccidioidomycosis.7ACS Environmental Au. Wildfire-Related Air Pollution and Infectious Diseases: Systematic Review and Meta-Analysis
The mechanism isn’t complicated: smoke particles irritate the lining of the airways and suppress local immune defenses, making it easier for viruses, bacteria, and fungi to gain a foothold. For LA residents, this means fire season and illness season can overlap in dangerous ways. When air quality advisories go up during a wildfire event, the risk isn’t just the smoke itself. It’s also the pneumonias, bronchitis, and fungal infections that follow in the weeks afterward. Wearing an N95 mask during heavy smoke days protects against both the particulate matter and airborne pathogens.
Valley Fever and Fungal Illness in the LA Region
Valley Fever, caused by a soil fungus called coccidioides, is one of the illnesses most specific to Southern California and the broader Southwest. The fungus lives in dry, alkaline soil and becomes airborne when the ground is disturbed by wind, construction, earthquakes, or farming. Once inhaled, the spores can cause an infection that ranges from a mild flu-like illness to severe pneumonia or, rarely, disseminated disease that spreads beyond the lungs.
The disease is most associated with California’s Central Valley, where it’s highly endemic, but cases occur throughout Southern California and the greater LA area, especially after dust storms or construction projects that churn up contaminated soil. Many people who get Valley Fever are initially misdiagnosed with a bacterial pneumonia or community-acquired respiratory infection because the symptoms overlap heavily: cough, fever, fatigue, chest pain. The difference is that Valley Fever doesn’t respond to antibiotics and requires antifungal treatment in moderate to severe cases.
The wildfire connection matters here too. The meta-analysis noted above found that medium-term wildfire smoke exposure increased hospitalizations for coccidioidomycosis by about 20%.8ACS Environmental Au. Wildfire-Related Air Pollution and Infectious Diseases: Systematic Review and Meta-Analysis The proposed explanation is twofold: fires disturb soil and loft fungal spores into the air, and the smoke simultaneously weakens respiratory defenses. If you’ve had a persistent cough for weeks after a fire event and antibiotics aren’t helping, Valley Fever is worth asking your doctor about, especially if you’ve been outdoors during dusty or smoky conditions.
Who Gets Hit Hardest in LA
Illness doesn’t hit every LA resident equally. During the first wave of COVID-19, an analysis of 698 worksite outbreaks investigated in Los Angeles County (excluding residential facilities and healthcare settings) found that nearly 60% occurred in just three sectors: manufacturing, retail trade, and transportation and warehousing. Manufacturing alone accounted for over a quarter of all workplace outbreaks and had the highest incidence rate of outbreak-associated cases. About 70% of people who got sick in these outbreaks reported Hispanic ethnicity.9PubMed Central. Industry Sectors Highly Affected by Worksite Outbreaks of Coronavirus Disease, Los Angeles County, California, USA, March 19-September 30, 2020
Those numbers reflected several overlapping realities: workers in manufacturing and warehousing often can’t work remotely, they share indoor spaces with poor ventilation, and they disproportionately come from communities with crowded housing and limited access to healthcare. While the specific outbreak dynamics of COVID-19 have shifted since 2020, the underlying vulnerabilities haven’t disappeared. Respiratory viruses of all kinds spread more easily in poorly ventilated indoor workplaces, and the workers most likely to be exposed are often the least likely to have paid sick leave.
Ventilation in LA’s built environment is a persistent problem beyond workplaces. A pilot study of portable classrooms in Los Angeles County found that ventilation rates were low, with a mean of 0.8 air changes per hour and some classrooms as low as 0.1 per hour. Compared to relevant standards, many of these classrooms were not providing adequate ventilation for the number of occupants.10PubMed. Evidence of inadequate ventilation in portable classrooms: results of a pilot study in Los Angeles County Portable classrooms are common in LA’s public schools, where space is tight and budgets for HVAC upgrades are limited. A classroom with ventilation rates that low is essentially a box where any respiratory virus brought in by one child has an excellent chance of reaching others.
What Rapid Tests Can and Can’t Tell You
When you feel sick in LA and grab a rapid antigen test from the drugstore, you’re almost certainly testing for COVID-19. These tests work, but their sensitivity depends heavily on timing. A study that compared rapid antigen test performance across SARS-CoV-2 variants found that when tested on the same day a person first showed a positive PCR result, the rapid test caught only about 15–22% of infections. By 48 hours after the first positive PCR, sensitivity climbed to roughly 45–50%. Among people who had been PCR-positive for at least 48 hours, sensitivity reached about 78–82%, with no statistically significant difference between Delta and Omicron variants.11PubMed Central. Comparison of Rapid Antigen Tests’ Performance Between Delta and Omicron Variants of SARS-CoV-2
A separate analysis confirmed that no significant differences appeared in the accuracy of rapid antigen testing across different SARS-CoV-2 variants. Diagnostic sensitivity increased when nasal swabs had lower cycle thresholds on PCR, which corresponds to a higher viral load, meaning rapid tests are best at catching infections when you’re most contagious.12JAMA Network Open. Accuracy of 2 Rapid Antigen Tests During 3 Phases of SARS-CoV-2 Variants
The practical lesson: a single negative rapid test on day one of symptoms doesn’t mean much. If you feel sick, test again 48 hours later. And remember that rapid tests only test for COVID-19. A negative result doesn’t rule out influenza, RSV, rhinovirus, or any other respiratory virus. If your symptoms are severe or you’re in a high-risk group, a PCR panel that tests for multiple pathogens gives a much clearer picture of what you’re dealing with.
Why LA Is a Hotspot for Introducing New Strains
Los Angeles International Airport is one of the busiest in the world, and that has direct consequences for disease introduction. Modeling research has shown that underestimating the volume of air travel between major population centers like Los Angeles and New York leads to models that underestimate how quickly an infectious disease can cross the country.13PLoS ONE. The Cost of Simplifying Air Travel When Modeling Disease Spread LA is a primary gateway for international arrivals from Asia and Latin America, and it serves as a domestic hub connecting the West Coast to virtually everywhere else in the country.
This connectivity means that when a new variant emerges overseas, LA is one of the first U.S. cities where it shows up. That’s exactly what happened with multiple SARS-CoV-2 variants, and it’s true for influenza strains, norovirus genotypes, and other pathogens as well. Southern California’s wastewater surveillance has picked up new SARS-CoV-2 mutations circulating in the community before they were widely detected through clinical testing.14PubMed Central. RNA Viromics of Southern California Wastewater and Detection of SARS-CoV-2 Single-Nucleotide Variants For residents, this means LA tends to be on the leading edge of new waves rather than catching them later from other parts of the country.
None of this makes LA uniquely dangerous, but it does mean the region is often an early-warning system for what the rest of the country will face a few weeks later. Paying attention to local surveillance data, staying current on vaccinations for the bugs that have them, and knowing that “what’s going around” in LA is usually several things at once are all genuinely useful habits for living in a city this connected to the rest of the world.
Protecting Yourself When Multiple Bugs Are Circulating
Because LA typically has several respiratory and gastrointestinal viruses circulating at the same time, the most effective protective strategies are the ones that work against multiple pathogens rather than just one. Handwashing with soap and water is the single most broadly effective measure: it works against both enveloped viruses like influenza and non-enveloped ones like norovirus that alcohol-based sanitizers struggle with. Staying home when you’re symptomatic keeps you from spreading whatever you have, even if you don’t know which virus it is.
For respiratory viruses specifically, ventilation matters enormously. Opening windows when weather allows, using portable HEPA filters in shared indoor spaces, and avoiding prolonged time in crowded, poorly ventilated rooms all reduce the concentration of airborne virus particles you’re exposed to. During fire season, when outdoor air quality is poor and opening windows isn’t practical, running a HEPA filter indoors serves double duty: it removes both smoke particles and airborne pathogens.
Vaccines exist for influenza, COVID-19, and now RSV (for older adults and, via maternal vaccination, for newborns). None of these vaccines eliminate infection entirely, but all reduce the likelihood of severe illness and hospitalization. For LA residents who commute on public transit, work in crowded settings, or have young children in school or daycare, the cumulative protection from staying up to date on available vaccines is substantial. There’s no vaccine for rhinovirus or norovirus yet, so for those, it comes back to the basics: wash your hands, stay home when you’re sick, and don’t assume a negative COVID test means you’re not contagious with something else.