It is not generally appreciated that it requires a certain amount of infection to induce disease. While an acute specific disease is the result of a reaction between two living organisms, the host and the parasite, nevertheless, in the vast majority of instances, it requires a certain amount of the infecting principle on the one hand and a certain degree of susceptibility on the other hand, to produce the fever and other constitutional disturbances we call "the disease."
These two factors are more constant than is commonly supposed. All individuals of a given species usually have about the same susceptibility to a particular infection. Exceptional individuals may occasionally show wide variations, some being especially susceptible, others unusually resistant. These exceptions are rare enough and conspicuous enough to emphasize the rule. Thus, almost all persons are about equally susceptible to smallpox, whooping cough, measles, tetanus, mumps, dengue, yellow fever, plague, scarlet