Following the boom in respiratory virology in the 1960s, species of rhinoviruses, coronaviruses, enteroviruses, adenoviruses, parainfluenza viruses and respiratory syncytial virus were added to influenza and measles viruses as causes of respiratory tract infection. In restricted patient groups, such as the immunocompromised, members of the family of herpesviruses including herpes simplex, cytomegalovirus, varicella-zoster virus, Epstein-Barr virus and human herpes virus 6 have also been associated with respiratory disease. This list of pathogens was extended last year with the discovery of a novel virus, the human metapneumovirus. More than 200 antigenically distinct viruses have been documented as causes of sporadic or epidemic respiratory infections in infants, children and adults. However, this varied and diverse group can be divided among six distinct families. Understanding some of the basic biology of these families gives an insight into possible strategies for diagnosis, control and therapy.