1. The infectivity of Bacillus abortus for guinea pigs is extremely great. It is computed that somewhat less than 100 organisms are required to infect most animals.
2. Occasional animals are encountered having much more resistance to Bacillus abortus infection than is possessed by the average. The most resistant animal encountered was infected by a number of organisms computed to be less than 10,000.
3. The principal effect on the character of the Bacillus abortus disease, caused by varying the size of the infecting dose, is a change in the time relations. Very small doses produce an infection of slower course than do larger doses, but the eventual results are practically the same.
4. Judging by the agglutination curves, guinea pigs inoculated with large or medium sized doses of Bacillus abortus generally begin to gain mastery over the disease about the 10th week after inoculation. At this time the agglutination curve begins to decline. As was first shown by Smillie, the numbers of bacteria in the body organs begin to decline at about the 4th week but the change is rather slow until about the 10th or 12th week when a rapid decline begins, coincidently with the fall in the agglutinin titer. When the inoculating dose is small the course of the disease may be much prolonged. The defensive forces of the body would appear to be quite inactive until a certain stage in the disease is reached.
5. The virulence of Bacillus abortus cultures for guinea pigs may be raised or lowered by appropriate passage through these animals.
6. Normal guinea pig blood will not agglutinate Bacillus abortus in a dilution of 1:10. Immune agglutinins are easily produced by inoculations with dead or living organisms. Agglutinins for Bacillus abortus are capable of passing the placental filter.
7. There appears to be little or no danger of normal guinea pigs becoming infected by association with diseased animals of their own sex. One instance is reported, however, of the infection of a male animal that had cohabited with infected females.
8. The susceptibility of guinea pigs to infection through ingestion of Bacillus abortus is relatively slight.