The Way Disease is Spread
Some diseases—for example, consumption and leprosy—are of indefinite but always prolonged duration; others, like measles, are short and sharp. In the case of the latter, for reasons we need not dwell on here, the body after an attack becomes, for a longer or shorter time, an unfit habitation for the microbes of that particular species. The rapid recovery which occurs in these “acute” diseases, indeed, implies the banishment of the microbes. The air-borne diseases—measles, influenza, smallpox, and the like, all of that acute type which confers immunity against subsequent attacks—are very infective, spreading through a susceptible population with great rapidity. Under favourable conditions the water-borne diseases also—cholera, dysentery, enteric fever, and the like—may spread very quickly. Chief amongst the earth-borne diseases is consumption. It is contracted chiefly in such dark, ill-ventilated, and crowded houses as are built by the inhabitants of cold and temperate climates.
The disease-producing microbes are an infinitesimal proportion of the total number of bacterial and protozoan species. In Nature it is not easy to find a speck of earth or a drop of water from which these minute living beings are absent. All decay, by means of which the dead bodies of plants and animals are returned to the soil, is due to them.
The Immense Antiquity of Diseases
It is a safe assumption that the microbes of human diseases have evolved from non-parasitic species. The niche they now occupy in Nature is the human body. Two things formed essential parts of this evolution—first, the microbes became capable of existing and multiplying for a shorter or longer period in the body; secondly, they evolved means of passing from one living body to another. The latter must have been the more difficult process. Under favourable circumstances several species of microbes—for example, those of putrefaction, which are ordinarily non-parasitic—are capable of entering the human body and becoming virulent; but, since they cannot secure passage from one individual to another, they die out, and their virulence is lost. Historical evidence renders it probable that all known human diseases are of immense antiquity, the so-called new diseases being merely newly-observed diseases. It appears probable, therefore, that, owing to constant persecution by disease, by continued survival of the fittest, humanity has grown so resistant that no species of microbe which has not undergone concurrent evolution is now able to establish itself as a regular parasite.
Obviously, since the microbes of human diseases draw their nutritive supplies from man, they cannot persist except amongst populations so crowded that they are able to pass from one individual to another in unending succession. When the succession fails, the disease dies out, and is not renewed, except from foreign sources. Microbic disease is never contracted in desert places far from human settlements, and even in modern times it is comparatively rare amongst nomadic tribes, and, seemingly, was quite unknown in Arctic regions and in many Pacific islands before its introduction by Europeans. These maladies, therefore, must have made their appearance only after men had peopled certain regions in considerable numbers.
Progress of Sanitary Science
On the other hand, we have no certain evidence that any well-established parasitic disease has ever completely died out. The chances are all against such an occurrence in the past. When once established as parasites, the microbes, owing to the constant growth of human population, found a constantly augmented food supply, and therefore constantly increased opportunities of reaching fresh fields of conquest. Sanitary science is still in its infancy. Preventive measures, and perhaps other agencies, have caused the disappearance of leprosy from several countries, but it is still prevalent in many quarters of the globe. Contagious diseases have spread very widely. Earth and air borne diseases have become endemic instead of merely epidemic. Consumption is always with us, and almost every child contracts measles, whooping-cough, chicken-pox, and common cold. Small-pox has been replaced by vaccination, which is merely modified small-pox. Malaria has spread but little during the historic epoch, but only because its microbes were already present in almost every place where the mosquitoes that convey it are able to exist.
THE DAYS OF THE PLAGUE IN LONDON