Hemophilus influenzae, type b thrives luxuriantly in the amniotic fluid of embryonated eggs inoculated by the intra-amniotic route on the 15th day of incubation. The effects of the establishment of the bacterial infection in the embryo are noted by the occurrence of death, bacteriemia or characteristic inflammatory lesions in the form of purulent sinusitis, pharyngitis, tracheo-bronchitis and meningo-encephalitis. These lesions may occur singly or in various combinations. The incidence and severity of disease manifestations in infected embryos depends on the proportion of encapsulated and virulent bacilli in the inoculum, the number of infectious doses and the growth rate of the bacteria in the surrounding amniotic fluid.

Combined viral and bacterial infection established by intra-amniotic inoculation with influenza C virus on the 14th followed by Hemophilus influenzae, type b on the 15th incubation day brings about a significant increase in the incidence and severity of disease manifestations in the embryos. Selective survival and marked acceleration of the growth rate of encapsulated and virulent elements of the bacterial population is promoted in the virus-infected embryos. The disease process appears to be entirely attributable to the bacterial component. There seems to be relatively little or no effect on influenza C virus by the accompanying proliferation of Hemophilus. The exact nature of the virus-induced influences which enhance the pathogenicity of the bacteria and favor the establishment of the infectious process under these circumstances remains to be determined.