In the preceding experiments an attempt has been made to work out on a mathematical basis certain laws of "test-tube infection" with special reference to the relation of dosage and activity of culture to resulting growth in favorable and unfavorable media. It has been shown that neither lag nor dosage is of great importance when dealing with a highly favorable medium, but that with an unfavorable medium both determine very directly the outcome of the inoculation.
Certain analogies have been drawn between these test-tube results and the observed phenomena of human and animal infection. If conditions are actually similar in the latter there is a simple explanation of several of the puzzles of epidemiology. It becomes intelligible why a carrier of pathogenic bacteria may reside in a community without producing infection of the susceptibles about him in spite of the fact that they actually acquire the organisms, why crowding results in the spread of infection, and why the introduction of unusually susceptible people into a community where carriers are present leads not only to infection of these susceptibles but to infection of the general population.
The experiments further raise the query whether alterations in natural vegetative activity of bacteria as well as specific alterations in virulence are of importance in explaining the spread of infection.
Finally, it should be said that the present experiments are not regarded as evidence for any hypothesis of human infection. They merely furnish interesting analogies. Further work with animals along the same lines is in progress.