Respiratory infection may aggravate chronic obstructive pulmonary disease. Viral respiratory infections may aggravate asthma, particularly in young individuals. Respiratory Syncytial virus and Rhinovirus dominate in children while, in adults, Influenza or Rhinovirus infections are most frequently concerned. Viral respiratory infections may also cause exacerbation of chronic bronchitis. Bacteria and their products scarcely play any part in asthmatic disease but may possibly aggravate chronic bronchitis and other forms of obstructive respiratory disease. In particular, Haemophilus influenzae and Streptococcus pneumoniae and bacterial endotoxin appear to be of significance. The mechanisms of the effects of viruses have several points of attack: Destruction of epithelium, release of mediators, potentiation of mediator-release and reduced beta-adrenergic function. Bacteria and their products may, similarly, cause bronchoconstriction and may, in vitro, release mediators and potentiate release of mediators.