Effect of cyclophosphamide on infections in mice caused by virulent and avirulent strains of influenza virus.

Induced immunosuppression with the drug cyclophosphamide was shown to convert the relatively harmless infection with an avirulent strain of Kunz influenza virus into a fatal pneumonic illness. The drug was also shown to increase the mortality of mice infected with low concentrations of a virulent variant of this strain, but it delayed the time of death of mice that were infected with high concentrations ofthe same variant. The probable roles of immune and inflammatory mechanisms in recovery from primary influenza virus infections are discussed.