Effect of influenza virus infection on susceptibility to bacteria in mice.

A model of combined infection was established with intranasal influenza virus and systemic Listeria monocytogenes infections of mice. Prior infection of mice with influenza virus markedly influenced resistance to subsequent challenge with L. monocytogenes. If mice were infected with influenza virus within the 24-hr period before challenge, a substantial increase in mortality was reflected by enhanced growth of Listeria in the spleen. If mice were infected with influenza virus three or five days before challenge, mortality was decreased, with an accompanying reduction in the growth of Listeria in the spleen. Thus, pulmonary infection with influenza virus has a major effect on susceptibility to systemic infection, exerting a depressive effect on host resistance in the first 48 hr and then causing a longer period of enhanced resistance.