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Page:Popular Science Monthly Volume 81.djvu/545

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suffering from typhoid fever, or to those of a chronic carrier of this disease, they may deposit the virulent bacteria upon food that later finds its way into our own bodies. Bacillary dysentery may be spread in the same fashion as well as many other gastro-enteric infections.

The house fly occurs practically throughout the entire civilized world and under all conditions is a continual menace to public health. In rural communities, however, where the proper disposal of waste matter of all kinds is most difficult, the importance of these insects is correspondingly increased.

The house fly is not the only insect which may act as a carrier of typhoid, for Dutton has shown experimentally that this may be spread by the bed-bug. These insects become infected through feeding on the blood of a person in the acute stage of the disease and for at least twenty-four hours retain the bacteria in a sufficiently virulent condition to inoculate a second person whom they may bite. That other biting insects such as fleas and mosquitoes may act in the same way is as yet unproved, but is by no means improbable.

Several other species of flies appear regularly in houses, but in far lesser numbers, and none exhibit to such a marked degree the peculiar tastes of the house fly, which wanders back and forth from filth to food, feeding on each in turn. In this method of feeding lies the danger of infection by house flies; they are equally fond of clean and filthy materials, and their frequent migrations from one to the other multiply their opportunities to pick up pathogenic organisms that may be later deposited upon foods.

The flea is another domestic insect which was looked upon only as a nuisance until it was shown that certain kinds of fleas are agents in spreading bubonic plague. The most terrible epidemics of which we have any historical record have been those of plague, or "black death." One swept from Egypt in the sixth century before the Christian era and invaded Europe and Asia, where it raged for sixty years. A similar one spread through the whole known world in the fourteenth century and is thought to have caused over twenty-five million deaths before it subsided.

In 1898 Simond suspected fleas as agents in the spread of plague and his suspicions have since been fully justified by Verjbitski and others. Plague is common to rats, certain other rodents and man, and is usually carried to man by the bites of fleas which have become infected from plague-stricken rats. The flea most commonly concerned is the rat flea, Læmopsylla cheopis. The transfer of plague bacteria is mechanical in nature, and other fleas, bed-bugs, etc., may also act as carriers, although far less commonly.

The plague bacilli (Bacillus pestis) appear only in fleas and bugs which have bitten affected persons or rats twelve to twenty-six hours