Separate groups of rats, mice, hamsters, and guinea pigs were caused to inhale virulent tubercle bacilli, of human or bovine strains, as single cells in fine droplet nuclei. Members of each of these eight host-parasite combinations were killed for study at stated intervals after infection.

For approximately 3 weeks after the bacilli were deposited in the lungs the progress of the infection, and the reaction of all species to it, followed a highly uniform developmental pattern. During the 4th week the rate and pattern of tubercle formation became distinctive for the species of host and the strain of parasite, but within any host-parasite combination this rate and pattern continued uniform for a time. The duration of this period of homogeneous response after the 4th week varied with the host-parasite combination, ranging from less than 5 weeks to more than 12 weeks after the induction of infection.

It is concluded that the highly uniform initial response is evidence that these animals do not differ in their inherent resistance to inhaled infection by the bacilli of human or bovine tuberculosis. Instead, they differ widely in their capacity to acquire resistance to these organisms, as shown by the variation in the later stages of the disease.

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