Page:Popular Science Monthly Volume 81.djvu/543

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ESS than fifteen years have elapsed since the scientific world entertained its first definite suspicion that certain human diseases might be spread through the agency of insects. Twelve have gone by since that suspicion became an established fact, and in this short space of time so much has been learned concerning the pernicious activities of these small animals in disseminating disease-causing organisms among man and the higher animals, that the science of preventive medicine can now be applied to many important diseases which were before utterly beyond its reach. Every year brings forth fresh evidence that insects are important factors in relation to public health, and adds to the list of diseases that are partially or entirely dependent upon certain insects for their spread.

A brief statement of the nature of communicable diseases and of the general habits of the kinds of insects that are implicated in carrying disease will serve to define roughly the field of medical investigation which is open to the entomologist. Communicable diseases are invariably due to parasitic organisms in the body which are capable of inducing similar symptoms in other persons or animals if transferred to healthy individuals from diseased ones. Many conditions modify the transfer of communicable diseases; some individuals are more easily infected than others; some may be immune as the result of a previous attack; and, on the other hand, the virulence of pathogenic organisms often varies greatly in accordance with conditions to which they have been subjected previously. A simple method of spread occurs with many diseases, for example typhoid fever and pulmonary tuberculosis With the former, the Bacillus typhosus which is the disease-producing organism, is present in the dejecta of an infected person and may find its way from these to food, carried by flies or otherwise; ingested by a healthy person, it may quite likely multiply and induce a second case of typhoid. With tuberculosis, the tubercle bacillus from desiccated sputum may enter the lungs of a healthy person with dust and there reproduce the disease. As we shall see later, certain insects are commonly associated with the spread of diseases of this type, although from the very nature of such diseases, insects are not exclusive factors, and may be referred to as contaminative carriers.

A second type of communicable diseases differs from the one just