Direct causes of disease.
The direct causes of disease may be due to the introduction into the human body of a specific microörganism which, if not met by the antagonistic agencies, finally pervades the whole system with its progeny or its virus. The microörganisms thus responsible for disease are commonly divided into two classes, namely, parasites and bacteria. In the first group are included those parasites that cause tapeworm, malaria, trichinosis, and hookworm; in the second group those bacteria that cause typhoid fever, cholera, erysipelas, diphtheria, and probably smallpox, measles, scarlet fever, chicken pox, and a number of others presumably similar.
Parasites as causes of disease.
The introduction of worms into the body must come either from impure drinking water, from impure food, or from the bites or stings of insects. When introduced into the body, those parasites that are inimical to man and produce abnormal conditions interfering with usual physiological functions may or may not develop further. In some cases, as in malaria, the very act of hatching the malarial brood is sufficient to throw the host on whom the brood will feed into a violent chill.
In other cases, as with the hookworm, while eggs are produced in the human body, they have no directly detrimental effect, the objectionable feature of their residence being due to the fact that the continual draught which they make upon the blood vessels of the intestine reduces the vitality, causing anæmia.
In other cases, as with the guinea worm, found in Africa and South America, the worm wanders from the stomach, which it enters toward the surface of the body, and finally breaks through, causing ulcers or abscesses.
In still other cases, as with that form of filaria which causes elephantiasis, the adult worm or the embryos are present in the lymphatics in such numbers as to interfere with circulation, causing the fearful swellings characteristic of the disease named.
Finally, in such cases as trichinosis and tapeworm, there is usually but little inconvenience to the human being harboring them, except when their number becomes very large. Then there may be diarrhœa, loss of appetite, and other digestive disturbances. The different tapeworms are generally responsible for nothing more than indigestion and nervousness. These latter parasites are, however, formidable in so far as their size is concerned. The mature pork tapeworm is about ten feet long, although the eggs, seen in the pork flesh, giving it its name of "measly," are only about a thousandth of an inch in diameter. The fish tapeworm, when mature, measures about twenty-five feet in length, while the beef tapeworm is about the same length. These worms can develop only in the bodies of the animals named, and find their way into the human body only through the medium of imperfectly cooked meat.
If proper precautions be taken in these directions, if only water is used for drinking which is known to be free from such parasites and their eggs, and if insects like mosquitoes and fleas are kept away by screening windows and doors, and if meat be always thoroughly cooked, the dangers of diseases from parasites will be reduced to a minimum.
Bacterial agencies.