Illustration of human metapneumovirus (hMPV) particles. This is a respiratory virus that infects almost all children by the age of 5. It is a type of paramyxovirus, consisting of RNA (ribonucleic acid) genetic material enclosed in a protein coat, or capsid, within a phospholipid envelope. The envelope is covered in protein spikes that are used to attach and penetrate their host cells. hMPV will only cause a mild respiratory tract infection in healthy adults but small children, elderly and immunocompromised individuals are at risk of severe disease and hospitalisation.