Acute lower respiratory tract infections (LRTIs) are a major worldwide health problem, particularly in childhood, and are ranked first among the conditions contributing to the global burden of disease. About 30–50% of acute LRTIs are viral in origin; of these, influenza and respiratory syncytial virus (RSV) are associated with the greatest disease burden in humans. Vaccination against circulating human influenza strains and the use of neuraminidase inhibitor drugs have improved the options for control of influenza, but as yet there are no successful vaccines or antiviral drugs for use against RSV infection. The recent emergence of the SARS coronavirus in the human population in 2003, with an ensuing global epidemic affecting more than 8000 individuals with a case fatality of about 10%, underlines the fact that respiratory viral infections of humans may originate in animals, and that many different influenza A viruses also occur naturally in animal reservoirs, representing a constant threat of zoonotic infections of humans and ensuing global pandemics. Avian influenza viruses have transmitted directly to humans from domestic poultry on several occasions in the last decade, and the current extensive burden of disease from avian influenza in South East Asia provides a real possibility for the emergence of a novel influenza virus pathogenic in humans.