It is generally known that poliomyelitis virus possesses a high degree of host specificity, man alone being susceptible to natural infection. With the exception of certain Old World species of monkeys, lower animals of all kinds are refractory to it. Even those species of monkeys which yield to experimental inoculation never contract the disease spontaneously from infected associates. They also are therefore endowed with a fair degree of natural resistance, differing from the natural state in man in that quite artificial conditions must be imposed to induce infection. This difference in susceptibility, incidentally, must be borne in mind in the application of experimental results to man.
The resistance exhibited by naturally refractory animals does not appear to be related to humoral antibodies, for the serum from such animals rarely inactivates the virus in vitro even when a high ratio of serum to virus is employed. On the contrary, it appears