The pathogenesis of infections of the mouse caused by virulent and avirulent variants of an influenza virus.

A virulent variant of the normally avirulent Kunz strain of influenza virus was obtained by serial passage in mice, and the pathogenesis of the infections caused by the two strains was studied. The virulence of the passaged variant did not appear to result from increased growth in lungs, from acquisition of resistance to non-specific inhibitors or from inadequate immunogenicity, but from its greater ability to replicate in alveolar cells. The apparent adequacy of defence mechanisms to protect against mucosal infection but their failure to protect against infection of alveolar cells in discussed.