It is well known that there is some form of “immunity” or resistance to insect bites. California, and particularly the San Francisco Bay region, abounds in fleas which do not encroach on the rights of most of the local population but which are a source of great misery to many newcomers until they acquire an “immunity.” This usually takes from several months to several years, and in rare instances is never acquired. The degree of selective biting has been vividly described by several of our subjects: “If there is one flea within a half mile radius, it will find me.”
A similar situation holds true in mosquito districts. Again it is newcomers who suffer most.
Our investigation was begun with the idea that immunization of people against insect bites (if possible) would materially decrease the chance of contracting diseases transmitted by the particular insect concerned.