How do I know what type of respiratory infection I have? 5:45

(CNN) -- Last winter was very intense in terms of respiratory viruses, dominated by waves of respiratory syncytial virus, influenza and covid-19. But just as it was coming to an end, a little-known virus that causes many of the same symptoms — lower lung infection, dry cough, runny nose, sore throat and fever — began to gain strength.

Human metapneumovirus (HMPV) cases spiked this spring, according to the U.S. Centers for Disease Control and Prevention's respiratory virus surveillance systems. Cases of virtus filled the intensive care units of hospitals for young children and the elderly, who are the most vulnerable to these infections. At its peak in mid-March, nearly 11% of samples tested tested positive for HMPV, a figure that is 36% higher than the average pre-pandemic seasonal peak of 7% test positivity.

However, most people who got infected probably didn't even know they had it. Patients are not usually tested outside of a hospital or emergency room. Unlike COVID-19 and influenza, there is no HMPV vaccine or antiviral drugs to treat it. Instead, doctors treat the seriously ill by attending to their symptoms.

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An underestimated threat

Studies show that the HMPV virus causes as much suffering in the U.S. each year as influenza and a closely related virus, RSV. A study of patient samples collected over 25 years revealed that it was the second most common cause of respiratory infections in children, behind RSV. A study conducted in New York over four winters revealed that it was as prevalent in hospitalized elderly as respiratory syncytial virus and influenza. Like those infections, HMPV can lead to intensive care and fatal cases of pneumonia in older adults.

Leigh Davidson contracted human metapneumovirus during a family celebration in early April. Two weeks later, he was coughing so violently that he couldn't talk on the phone.

"I couldn't say more than a couple of words," said Davidson, 59, an entertainment attorney in Baltimore. "I was coughing violently, violently, to the point that I literally almost threw up."

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Her cough was so constant and deep that she was convinced that she had finally contracted the coronavirus after having managed to avoid it throughout the pandemic. But he took six rapid covid-19 tests, and all came back negative.

Davidson is immunodeficient, so he has been cautious throughout the pandemic. Worried about pneumonia, she had an X-ray at a radiological clinic near her home and was told she was clean.

However, her doctor was not satisfied and sent her to the emergency room for further tests. Blood tests determined he had metapneumovirus.

"I was like, 'What? Because it sounds very serious," Davidson says. "I've never heard of it."

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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 needed mechanical ventilation, but did not test positive for any known pathogens.

The researchers grew the samples in various cell types from monkeys, chickens and dogs, and then 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 cause respiratory diseases such as measles, mumps and respiratory syncytial virus (RSV).

Closer examination of the virus' genes showed a close relative - avian metapneumovirus, which infects birds. The new virus was dubbed human metapneumovirus. Scientists believe it likely jumped from birds to humans at some point and evolved from there.

When the researchers analyzed blood samples from 72 patients that had been stored since 1958, they all showed signs of exposure to the mysterious virus, indicating that it had been circulating in humans, undetected, for at least the previous half-century.

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Doctors and patients in the dark

Respiratory infections are the leading cause of infant death worldwide and the leading cause of hospitalization of children in the United States, but scientists don't know the cause of many of them, says Dr. John Williams, a pediatrician at the University of Pittsburgh who has spent his career researching vaccines and treatments against HMPV.

Williams says that in the 1950s and 1960s extensive epidemiological studies were conducted on the causes of respiratory infections.

"Basically, they could only identify a virus in people about half the time. So the question was, 'Well, what about the other half?'" he explains. Human metapneumovirus does not represent all unknown viruses, but it does represent a significant proportion: almost as many cases as RSV or influenza.

But no one knows him. Williams calls it "the most important virus you've ever heard of."

"Those are the three main viruses," he said. "Those are the big three in children and adults, the most likely to take people to the hospital and cause serious illness, the most likely to raze nursing homes and make older people really sick and even die."

Since human metapneumovirus testing is rarely done outside of hospitals, it is difficult to know the true burden of disease.

Blood tests show that most children have had it before age 5.

A 2020 Lancet Global Health study estimated that among children under 5, there were more than 16 million HMPV infections in 2018, more than 600,000 hospitalizations, and more than 16,000 deaths.

However, the infection results in weak or incomplete immune protection, and humans are reinfected throughout their lives.

Companies are working on vaccines against it. Moderna, maker of the COVID-19 mRNA vaccine, just completed a preliminary study of an mRNA vaccine against HMPV and parainfluenza, according to the clinicaltrials.gov website.

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CDC recommends that doctors consider testing for HMPV virus in winter and spring, when it typically peaks.

According to Williams, doctors don't test primarily because of ignorance of the virus, but also because a test probably wouldn't change the care they would pay to the patient. It would help them rule out other causes that do have specific treatments, such as covid or influenza.

Davidson said the human metapneumovirus caused him a severe attack of bronchitis. She was briefly admitted to the hospital for observation. She eventually got better, but she was sick for a month.

She'd had respiratory infections before, of course, but she's especially glad to be on the other side of the human metapneumovirus, she said. "This was really the worst thing I had ever experienced."

Respiratory diseases