Susceptibility of host to infection. Many animal and human races differ in their susceptibility toinfection. For example Negroes whose red cells contain hemoglobin S are relatively immune to malaria. Man and the guinea-pig may be infected by human or bovine strains of the tubercle bacillus, but are unaffected by the avian strain. Rabbits are very susceptible to experimental infection with the bovine strain, rabbits are very susceptible to experimental infection with the bovine strain, though they seldom if ever contract tuberculosis naturally. Birds on the other hand are susceptible only to the avian variety. When measles was first introduced by traders into the Fiji Islands there was a very high mortality rate at all ages.