New respiratory viruses of humans.

Acute respiratory viruses are a major cause of morbidity and mortality in humans worldwide and most acute respiratory infections are caused by viruses. Many of these viruses cause the highest burden of disease in specific risk groups such as young infants, the elderly, and immune-compromised individuals. Although the most important respiratory viruses of humans have been identified in the last century, in the past decade about a dozen "new" viruses have been discovered that may cause a high burden of acute respiratory disease in humans. Not only viruses were discovered that must have been with humans for many decades or centuries, such as human metapneumovirus and 2 different human coronaviruses, but also viruses that are truly new for humans and have emerged as a result of recent interspecies transmissions from other mammalian or avian reservoirs. The latter include highly pathogenic avian influenza viruses, severe acute respiratory syndrome (SARS) coronavirus, and Nipah virus. The discovery, etiologic role, and burden of disease caused by these infections are described.