Human metapneumovirus (HMPV), a virus that causes respiratory illness, was first discovered in 2001, although humans have been infected with it for at least 50 years. HMPV may cause upper respiratory illness or no symptoms at all in healthy adults, but older adults, adults with asthma, and children may be at risk of more serious illness. HMPV is a leading cause of viral lower respiratory infection (LRI) in children, so finding a vaccine for this virus could substantially reduce the instances of childhood respiratory illnesses.