The accomplishment of an infection, Zinsser1 writes, is not determined merely by the fact that a microorganism of a pathogenic species finds lodgment in or on the body of a susceptible individual, but it is further necessary that the invading germ shall be capable of maintaining itself, multiplying and functionating within the new environment. An infection, then, or an infectious disease, is the product of the two factors, invading germ and invaded subject, each factor itself influenced by a number of secondary modifying circumstances, and both influenced materially by such fortuitous conditions as the number or dose of the infecting bacteria, their path of entrance into the body, and the environmental conditions under which the struggle is maintained.
This struggle between the host and the invading bacteria represents a situation in which no adaptation has taken place. The conflict may result in a local or systemic manifestation which constitutes
RESISTANCE TO INFECTION. JAMA. 1916;LXVII(17):1230. doi:10.1001/jama.1916.02590170038014