A respiratory virus that most people have never heard of was to blame for many hospitalizations and illnesses this spring, health officials say. The human metapneumovirus, or HMPV, causes symptoms similar to COVID-19 or RSV, but doctors don’t usually test for it outside of a hospital setting, meaning many people likely had no idea they had it. The Centers for Disease Control and Prevention reports that cases of HMPV surged about 36 percent in mid-March, with 11 percent of all specimens tested indicating the virus. Dr. John Williams, a pediatrician at the University of Pittsburgh, called HMPV “the most important virus you’ve never heard of” in comments to CNN, noting that it accounts for about the same number of illnesses as RSV and the flu. The virus can lead to complications and intensive care for vulnerable patients, but in most cases the symptoms of cough, fever, nasal congestion, and shortness of breath will resolve without treatment after several days.