A respiratory virus that doesn’t have a vaccine or a specific treatment regimen is spreading in some parts of California — but there’s no need to sound the alarm just yet, public health officials say.

A majority of Northern California communities have seen high concentrations of human metapneumovirus, or HMPV, detected in their wastewater, according to data from the WastewaterScan Dashboard, a public database that monitors sewage to track the presence of infectious diseases.

A Los Angeles Times data analysis found the communities of Merced in the San Joaquin Valley, and Novato and Sunnyvale in the San Francisco Bay Area have seen increases in HMPV levels in their wastewater between mid-December and the end of February.

HMPV has also been detected in L.A. County, though at levels considered low to moderate at this point, data show.

While HMPV may not necessarily ring a bell, it isn’t a new virus. Its typical pattern of seasonal spread was upended by the COVID-19 pandemic, and its resurgence could signal a return to a more typical pre-coronavirus respiratory disease landscape.

Here’s what you need to know.

What is HMPV?

HMPV was first detected in 2001, according to the U.S. Centers for Disease Control and Prevention. It’s transmitted by close contact with someone who is infected or by touching a contaminated surface, said Dr. Neha Nanda, chief of infectious diseases and hospital epidemiologist for Keck Medicine of USC.

Like other respiratory illnesses, such as influenza, HMPV spreads and is more durable in colder temperatures, infectious-disease experts say.

Human metapneumovirus cases commonly start showing up in January before peaking in March or April and then tailing off in June, said Dr. Jessica August, chief of infectious diseases at Kaiser Permanente Santa Rosa.

However, as was the case with many respiratory viruses, COVID disrupted that seasonal trend.

Why are we talking about HMPV now?

Before the pandemic hit in 2020, Americans were regularly exposed to seasonal viruses like HMPV and developed a degree of natural immunity, August said.

That protection waned during the pandemic, as people stayed home or kept their distance from others. So when people resumed normal activities, they were more vulnerable to the virus. Unlike other viruses, there isn’t a vaccine for human metapneumovirus.

“That’s why after the pandemic we saw record-breaking childhood viral illnesses because we lacked the usual immunity that we had, just from lack of exposure,” August said. “All of that also led to longer viral seasons, more severe illness. But all of these things have settled down in many respects.”

In 2024, the national test positivity for HMPV peaked at 11.7% at the end of March, according to the National Respiratory and Enteric Virus Surveillance System. The following year’s peak was 7.15% in late April.

So far this year, the highest test positivity rate documented was 6.1%, reported on Feb. 21 — the most recent date for which complete data are available.

While the seasonal spread of viruses like HMPV is nothing new, people became more aware of infectious diseases and how to prevent them during the pandemic, and they’ve remained part of the public consciousness in the years since, August and Nanda said.

What are the symptoms of HMPV?

Most people won’t go to the doctor if they have HMPV because it typically causes mild, cold-like symptoms that include cough, fever, nasal congestion and sore throat.

HMPV infection can progress to:

• An asthma attack and reactive airway disease (wheezing and difficulty breathing)
• Middle ear infections behind the ear drum
• Croup, also known as “barking” cough — an infection of the vocal cords, windpipe and sometimes the larger airways in the lungs
• Bronchitis
• Fever

Anyone can contract human metapneumovirus, but those who are immunocompromised or have other underlying medical conditions are at particular risk of developing severe disease — including pneumonia. Young children and older adults are also considered higher-risk groups, Nanda said.

What is the treatment for HMPV?

There is no specified treatment protocol or antiviral medication for HMPV. However, it’s common for an infection to clear up on its own and treatment is mostly geared toward soothing symptoms, according to the American Lung Assn.

A doctor will likely send you home and tell you to rest and drink plenty of fluids, Nanda said.

If symptoms worsen, experts say you should contact your healthcare provider.

How to avoid contracting HMPV

Infectious-disease experts said the best way to avoid contracting HMPV is similar to preventing other respiratory illnesses.

The American Lung Assn.’s recommendations include:

• Wash your hands often with soap and water. If that’s not available, clean your hands with an alcohol-based hand sanitizer.
• Clean frequently touched surfaces.
• Crack open a window to improve air flow in crowded spaces.
• Avoid being around sick people if you can.
• Avoid touching your eyes, nose and mouth.

Assistant data and graphics editor Vanessa Martínez contributed to this report.

Red dwarfs make up the vast majority of stars in the galaxy. Such ubiquity means they host the majority of rocky exoplanets we’ve found so far – which in turn makes them interesting for astrobiological surveys. However, there’s a catch – astrobiologists aren’t sure the light from these stars can actually support oxygen-producing life. A new paper, available in pre-print on arXiv, by Giovanni Covone and Amedeo Balbi, suggests that they might not – when it comes to stellar light, quality is just as important as quantity. And according to their calculations, Earth-like biospheres are incredibly difficult to sustain around red dwarfs.

Their argument is based on the concept of exergy – a measurement of the maximum amount of useful work that can be extracted from a radiation field. In other words, it measures the thermodynamic quality of the light, not just the raw energy contained in it. Typically, when measuring the “habitable zone” of stars, astrobiologists look at the total number of photons, specifically in the visible light range between 400 and 700 nanometers of wavelength.

So what “useful work” does light do on exoplanets? Perhaps the most important is breaking apart water. This process, known as “water oxidation” is a kinetic bottleneck in the process of photosynthesis, and creates the oxygen expected to be seen in biosignatures. However, to do this, biological systems require a significant amount of kinetic energy to perform this chemical reaction. And red dwarfs have two strikes against them when it comes to providing that energy.

Fraser talks about habitable planets around Red Dwarfs

Red dwarfs are cool, and their light is heavily red-shifted into the infrared. Not enough of their photons pack enough energy to reach the threshold needed to split water. But even the ones that do have a smaller percentage of their energy that can actually be converted into useful chemical work. This one-two combination puts a huge dent in the potential of oxygenic life forming around red dwarfs. By comparison, the exergy available to drive water oxidation around Sun-like stars is around five times higher.

Astrobiologists are an optimistic bunch, though, so their immediate response to this concern would be – maybe life evolved around those stars to adapt to these higher infrared environments. Could they use longer, lower-energy infrared wavelengths under the skies of a red dwarf? The short answer is no, due to something called the red limit. This is the longest wavelength of light capable of supporting photosynthesis. The authors argue that this isn’t a set value, but an emergent property determined by a star’s spectrum, the planet’s atmosphere, and a targeted chemical reaction – in this case the water oxidation.

They estimate that for red dwarfs the red limit is 0.95 um, whereas for Sun-like stars its closer to 1.0 um. In practice, that means life cannot simply shift their primary absorption bands deeper into the near-infrared to adapt to their less powerful star. Another concern has to do with the evolution of life on one of these planets. Anoxygenic bacteria can effectively harvest infrared light. If allowed to proliferate, they could out-compete oxygenic bacteria, and the world would never experience a “Great Oxidation Event” equivalent to what happened on Earth. Without copious amounts of oxygen in the atmosphere, multicellular life would be severely hindered, if not outright eliminated altogether.

Fraser has a few videos on this topic, showing that there’s been an ongoing debate.

Taking all of this into account paints a bleak picture for the possibility of life around red dwarfs. But let’s not rule it out entirely. Currently, the Earth’s biosphere only uses about three orders of magnitude below the maximum thermodynamic – proof that life itself is wildly inefficient. But even so, the conditions surrounding red dwarfs that would be favorable for life are likely extremely rare. This paper proves that our time searching for an oxygen-rich alien forest might be better spent around stars like our Sun, rather than chasing the statistical rarity of a flourishing biosphere surrounding a red dwarf.

Learn More:

G. Covone & A. Blabi – Photosynthetic exergy I. Thermodynamic limits for habitable-zone planets

UT – Red Dwarfs Are Too Dim To Generate Complex Life

UT – Habitable Zone Planets Around Red Dwarfs Aren’t Likely To Host Exomoons

UT – New Research Suggests Red Dwarf Systems are Unlikely to Have Advanced Civilizations