Studies with neurotropic viruses (1–3) have shown that younger animals are more susceptible to virus infection than older animals. With other virus groups, the evidence has been more limited. There are many references to the use of young mice in studies on influenza, but little information is given concerning the reasons for this choice. Sarracino and Soule (4) observed that 4–6 week old mice succumbed as much as 10 days before older animals when inoculated with the virus of influenza. The present study was undertaken in order to ascertain the effect of age upon susceptibility to influenza virus. This effect of age upon susceptibility to that infection is of considerable interest, inasmuch as it is felt that the observed differences may possibly reflect information concerning the mechanism of host-parasite relationships.

Materials and Methods. Mice: The Swiss mice employed in this study were all received from one dealer2 and were approximately the same age (3 weeks) on arrival.

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