CNN  — 

The past winter was a heavy one for respiratory viruses, dominated by surges of RSV, influenza and Covid-19. But just as it was winding down, a little-known virus that causes many of the same symptoms – a lower lung infection, hacking cough, runny nose, sore throat and fever – was just picking up steam.

Cases of human metapneumovirus, or HMPV, spiked this spring, according to US Centers for Disease Control and Prevention’s respiratory virus surveillance systems. It filled hospital intensive care units with young children and seniors who are the most vulnerable to these infections. At its peak in mid-March, nearly 11% of tested specimens were positive for HMPV, a number that’s about 36% higher than the average, pre-pandemic seasonal peak of 7% test positivity.

Most people who caught it probably didn’t even know they had it, however. Sick people aren’t usually tested for it outside of a hospital or ER. Unlike Covid-19 and the flu, there’s no vaccine for HMPV or antiviral drugs to treat it. Instead, doctors care for seriously ill people by tending to their symptoms.

An underestimated threat

Studies show that HMPV causes as much misery in the US each year as the flu and a closely related virus, RSV. One study of patient samples collected over 25 years found that it was the second most common cause of respiratory infections in kids behind RSV. A study in New York conducted over four winters found that it was as common in hospitalized seniors as RSV and the flu. Like those infections, HMPV can lead to intensive care and fatal cases of pneumonia in older adults.

Diane Davison caught human metapneumovirus during a family celebration in early April. Two weeks later, she was coughing so violently, she couldn’t talk on the phone.

“I couldn’t get out more than a couple of words,” said Davison, 59, an entertainment lawyer in Baltimore. “I would go into violent, violent coughing to the point where I was literally almost throwing up.”

Her cough was so constant and deep, she was convinced she had finally caught the coronavirus after managing to avoid it throughout the pandemic. But she took six rapid tests for Covid-19, and all came back negative.

Davison is immunocompromised, so she has been cautious throughout the pandemic. Concerned about pneumonia, she got a X-ray from a radiology clinic near her home and was told it was clear.

Her doctor wasn’t satisfied, however, and sent her to an emergency room for more testing. Blood tests determined that she had HMPV.

“I was like, ‘what?’ Because it sounds really dire,” Davison said. “I’ve never heard of it.”

Human metapneumovirus was discovered by Dutch virus hunters in 2001. They had 28 samples from children in the Netherlands with unexplained respiratory infections. Some of the children had been very ill and required mechanical ventilation, but they didn’t test positive for any known pathogens.

The researchers cultured the samples in various types of cells from monkeys, chickens and dogs, and then they looked at the cultures under an electron microscope. They saw something that seemed structurally related to the paramyxoviridae family, a group of viruses known to give people respiratory disease like measles, mumps