from James E. Crowe, Jr. and John V. Williams writing in The Biology of Paramyxoviruses:
The metapneumoviruses were discovered more recently than most paramyxoviruses, because of the difficulty in isolating lives viruses in cell culture from clinical materials. Nevertheless, rapid progress has been made in identifying the salient features of the epidemiology and biology of these viruses. Remarkably, human metapneumovirus appears to be one of the most common causes of severe lower respiratory tract illness in infants and children. The virus is also a major cause of illness in the elderly. Avian metapneumovirus also was discovered relatively recently and is a major cause of disease in birds, especially in commercial poultry. Molecular biologists have defined the distinguishing features of the viral genomes, and animal models have been developed that allow detailed studies of pathogenesis, immune mechanisms, and protective efficacy of candidate vaccines.
Further reading: The Biology of Paramyxoviruses