Its regurgitated fluid commonly swarms with bacteria, microbes, and the like. Imbibed bacteria are not inevitably killed in the digestive process of the fly, for its excrement has been found to abound with well recognisable infective germs. In the market, the shop, the larder, and on our tables, the house-fly seeks every opportunity of befouling and contaminating human food.
The varieties of micro-organisms are multitudinous, doubtless many more in number than the microscopist and the bacteriologist expert have yet isolated and registered as capable of identification. Granted that the majority of these are non-pathogenic to humanity, still a formidable number, including some which are very generally disseminated, are virulently pathogenic, and many are suspect. There is no need to give a list of all the infectious diseases which man and beast are liable to contract, but the germs of nearly all may be carried from place to place, from creature to creature, and from person to person, through the intervention or agency of the house-fly. The medical profession are convinced that infantile mortality from epidemic diarrhœa must be attributable to summer flies.
In the matter of food which becomes fly-infected after having been cooked, or of food like milk, butter, and fruit, which are consumed raw, it should be known that a single pathogenic germ of ultra-microscopic dimensions, having obtained lodgment in the body, may there multiply and originate a fatal disease. On the other hand, raw meat which has been infected may, after the bacteria have been all killed by cooking, contain excreted poison in deleterious quantity. The decomposition of infected meat begins ten or twelve hours before the bad odour is perceptible.
Fortunately the omnipresent germs which most commonly deteriorate our food are not very actively deleterious, or are only slightly debilitating; yet wherever such less obnoxious germs get lodgment, there the ready prepared and most favourable breeding place for the worser kind is to be found. The various species of these evil things are not always exterminating competitors; they sometimes flourish in company, and dwell together, like the seven devils within the exorcised and sane man after his relapse, as mentioned in the Scripture.
That food gets fly-blown and maggot-infected is a very disgusting fact, but the plainly visible result is of little hygienic significance apart from the more concealed facts of the fly-borne conveyance of zymotic diseases.
Internal protozoal parasites and parasitic worms breed in and are disseminated by the house-fly; so also are the fungic spores of fermentive yeasts, of moulds, and the like, but these latter are mainly disseminated by mere air currents. The eggs of tape-worms and the like are carried by dung-frequenting flies to food, especially to semi-putrid food devoured by dogs and pigs.
Some of the skin-piercing and blood-sucking flies are pestiferous in a more direct way than any of the tribes of filth and sweat-flies. They are the usual or suspected agents whereby anthrax, cattle-plague, swine fever, glanders, and other diseases are spread far and wide. Some of these last blood-sucking flies will travel with and on the bodies of transported animals for long distances; of course there can be no doubt also as to the capability of disease dissemination by the direct independent flight of flies to long distances with favouring weather and breeze. Such evils are prevalent throughout the temperate zones, but circumstances are far worse in the tropics, where Glossina morsitans, considered by some to be a near relation of our Stomoxys calcitrans, transmits the microscopic trypansomes which cause the devastating "sleeping sickness" of mid-Africa. This last reference, and other discoveries of the fly-borne germs of recurrent fevers, should bring into prominent notice a very pertinent fact; which has not yet received adequate scientific investigation. All the bites of our common blood-sucking insects, flies, gnats, midges, fleas, etc., are each kind of them wounds, sometimes very inflammatory, sometimes but little or not at all so; furthermore, the worst inflammatory wounds not uncommonly show feverish symptoms of a well marked periodic character, quiescent intervals being followed by revived inflammation in the same spot. These facts almost prove, or at least strongly suggest, an explanation that in the latter cases the source of pathogenic trouble is of a microbic character, and that periodic recurrence of feverish and inflammatory symptoms is the time of the spore-swarming of breeding microbes; the difference between these latter and those of a more severe and often fatal kind in the tropics being that the one class finds the human body a fit host in which to multiply but the other class does not, and accordingly the latter grow weaker until their breed dies out. Similar effects can be observed in cattle and other animals, but all creatures, after suffering much from fly-bites at first, afterwards become for a time more or less immune.
CHAPTER X
REMEDIAL MEASURES
We have seen in Chapter VIII that the checks which Nature has imposed upon the prolific breeding of the house-fly have been insufficient to protect civilised mankind from ancient times continuously up to the present day. This defect need now no longer be endured; but, alas, communities and individuals are ever slow to be warned, and averse to practise newly advised methods of sanitation. In few other directions is there greater promise of advancement in general public health and comfort than by preventive measures against the breeding of the house-fly. Effective measures comprehensible to all who consider the subject are so easy of application, that, if universally carried out, the house-fly might become a rare insect in a very few years' time. It is, however, of fundamental importance that the public should be made to comprehend the case; else the power necessary for enforcing suitable regulations by the local authorities will not be obtainable.