Page:Popular Science Monthly Volume 81.djvu/42

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HE rapid progress during recent years in the knowledge and treatment of human diseases has been marked by a number of discoveries so important and fundamental in their nature that intelligent people everywhere are paying homage to the discoverers, some of whom have given the best part of their lives for the benefit of others. In no line of scientific activity are the results of recent discovery more far reaching or have they a more important bearing on the daily lives of men, women and children than in medical entomology—or the relation of insects to the transmission of human diseases.

The diseases that may be spread by insects are of course those that are commonly known as germ diseases, some of which are regarded as contagious or infectious. They are in some cases blood diseases, and affect the entire system, while in others perhaps only one part of the body, or certain organs, are involved. Some, like typhoid fever and tuberculosis, are caused by bacteria, the lowest forms of plant life, and others, like scarlet fever and malaria, by protozoa, which are animals low in the scale of classification. It is manifestly impossible in this paper to place before you all of the evidence, or even brief descriptions of the various studies and experiments which enabled the investigators finally to obtain the facts that make up our present knowledge of the subject. I shall therefore mention only a few of the strategic points and striking illustrations, hoping that these may be sufficient to show the importance of the subject and the necessity for action, and to enlist your interest in it.

The agency of insects in the spread of human diseases is of two sorts—(1) mechanical carriers, (2) essential hosts.

To the first group belongs the common house fly, and whatever germs adhere to the body, feet, tongue or wings of the fly in its perambulations in and over filth, or those that are swallowed by it, may be deposited on food or in other places in such manner as to cause infection.

The mosquito is a good example of the second group. One species, Anopheles maculipennis, is one of the necessary hosts in the development of the malarial parasite. Man is the other host, and in the blood