Doctors are warning people to remain vigilant as seasonal flu viruses that can cause pneumonia and bronchitis spread in California wastewater and across the country.
High levels of human metapneumovirus (HMPV) have been detected in dozens of cities in Northern California, according to the public database WastewaterScan Dashboard.
Redwood City has the highest concentration, according to the dashboard. Higher concentrations have also been found in the San Francisco Bay Area and Napa Wine Country.
In other parts of the country, HMPV levels are declining. The map on the dashboard shows that California and the Northeast Coast have the highest HMPV levels.
Cases have been on the rise nationwide since October 2025, according to the U.S. Centers for Disease Control and Prevention. The agency noted that HMPV typically spreads during the winter and spring.
High concentrations of human metapneumovirus are present in California wastewater. Cases have been increasing in the United States since October 2025 (Getty Images)
Dr. Matthew Binnick, director of the Clinical Virology Laboratory at the Mayo Clinic, previously explained: “In late winter and early spring, it accounts for 5 to 10 percent of all respiratory infections that we diagnose in the United States. So it’s definitely there.”
Dr. Monica Gandhi, an infectious disease expert at the University of California, San Francisco, said that with the COVID-19 season off, “other viruses like influenza and HMPV are getting opportunities.” SF door when asked about the state’s cases.
The viruses “compete with each other,” she said. “When COVID-19 surged, we saw very low rates of other common cold viruses.”
HMPV infection typically causes cold-like symptoms but only lasts a few days to a week. But experts warn it can also lead to more serious symptoms, such as middle ear infections, life-threatening asthma attacks and barking coughs.
“The main symptoms are cough, runny nose, fever. It also tends to make you wheeze, especially if you have underlying asthma,” Dr. David Weber, medical director and professor of infectious diseases at UNC Health, told North Carolina WTVD.
In rare cases, the infection can lead to death in more vulnerable patients, including infants, the elderly and immunocompromised people, researchers said.
The same goes for other respiratory viruses that circulate across the country, such as influenza and respiratory syncytial virus.
Unlike influenza and respiratory syncytial virus, there is no vaccine to prevent HMPV infections, and these infections are diagnosed through laboratory testing.
According to the Mayo Clinic, almost everyone will be infected with at least one HMPV in their lifetime. The Cleveland Clinic notes that if you’re over the age of five, you’ve probably had it at least once.
A 2025 report showed that from 2016 to 2019, there were an estimated 12 hospitalizations per 100,000 U.S. adults each year.
A chart from the Centers for Disease Control and Prevention showing weekly positive HMPV test results in the U.S. from 2024 through this month (CDC)
Most people recover with rest and fluids, and Weber says over-the-counter medications can relieve symptoms.
Hospitals may provide patients with oxygen masks, steroids to reduce the inflammation causing symptoms, or intravenous fluids to help prevent symptoms from getting worse, according to the Cleveland Clinic.
Respiratory viruses can be spread through direct contact, sneezing, coughing, or touching infected surfaces.
To prevent infection, the CDC recommends that people wash their hands regularly with soap, clean surfaces, and stay home if they feel sick.
HMPV was discovered in 2001, but research shows the virus has been circulating since the 1970s.
