In influenza the combined virus-bacterial pneumonia is approximately three times more common than primary viral pneumonia1. The bacteria most commonly involved are Staphylococcus aureus, Streptococcus pneumoniae and Haemophilus influenzae. S. aureus co-infection is reported to have a fatality rate of up to 42% (ref. 2). It is thought that virus infection in the respiratory tract favours growth conditions for bacteria. In this letter data are presented which show that some S. aureus strains secrete a protease which exerts a decisive influence on the outcome of influenza virus infection in mice by cleavage activation of the virus haemag-glutinin.
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