Part 8

From these and similar facts it is inferred, that when putrescent or decomposing organic matter is introduced into the blood it acts as a poison and produces the phenomena of fever, and that all the predisposing causes of epidemics act in this way—by overcharging the blood with the products of decomposing organic matter.

Strictly speaking, however, all that we really know is this—that where certain conditions exist, epidemics break out and spread; that where those conditions do not exist, epidemics do not break out and spread; and that where those conditions did exist, but have been removed, thereupon epidemics cease.

We call those conditions Causes, Predisposing or Localizing Causes, but how they act, whether by accumulating decomposing organic matter in the blood, or in what other way, we have no certain knowledge.

One further fact however is ascertained, that where any one of these predisposing causes is present, epidemics break out and spread just as readily as when all are present together.

Where there is overcrowding alone, for example, epidemics break out and spread. Where there is decomposing filth alone, epidemics break out and spread; and so of the whole number. The removal of one of these causes, therefore, or the removal of two or three of them, will not suffice for safety; every one must be removed before there can be safety.

This we know; all beyond this is conjecture, but as to the most probable of these conjectures, some who have thought on this subject believe that the preponderance of evidence justifies the conclusion that the predisposing causes may themselves become efficient causes; that instances in which they actually do so, are constantly passing before our eyes; that it is practicable to manufacture fever and even epidemic fever to any amount by placing a population under certain known conditions; that it is practicable to prevent the outbreak of epidemics altogether by placing the population under certain other conditions; that the prevalence of the predisposing causes in particular localities, in certain intensities, is sufficient to produce local epidemic outbreaks; that the prevalence of such causes in such intensities, joined to some general conditions of the atmosphere, such as the meteorological conditions which have been enumerated, particularly those which favour the accumulation and concentration of the products of organic decomposition, are all that is required to engender wide-spread epidemics. Those who adopt this view contend that the existence of a primary cause as a distinct and separate entity is not necessary to account for the phenomena.

The more common opinion however is, that joined to the predisposing causes there must always be present a primary cause, having a distinct existence, capable of travelling from one part of the globe to another; capable of spreading over any space however extended, or of confining itself to any space however small—a district, a street, a house, a room.

Footnote 10:

See Baltimore case, p. 78.

It is urged that though we are unacquainted with the physical form or chemical properties of this body, this is no reason why we should not understand its force as a special agent in the production of disease, just as we know the forces of other physical bodies, though not their nature.

The existence of such a body being assumed, it is conceived that it exists not in a gaseous but in a liquid state. It is supposed that it cannot exist in a gaseous state because a gas is readily diffused and dissipated; because when organic matter is reduced to a gaseous state, it has passed from the organic into the inorganic kingdom, and there is no evidence that the elementary bodies belonging to this kingdom are capable of producing any form of fever; and because there is indubitable evidence that organic matter in a recent state of putrescence—the more recent the more potent—is capable of producing the most deadly forms of fever. From these considerations it is conjectured that the primary cause, whatever it be, is some subtle fluid which has not wholly lost its organic composition, and that it consists of particles of extreme minuteness, capable of attaching itself to the surfaces of other bodies, and even of increasing under favourable circumstances.

It is further thought that this body is not equally diffused through the atmosphere, but is only partially distributed, and that this accounts for the local distribution of epidemics, and for their occasional absence from places which apparently present all the conditions favourable to their development.

Lastly, the opinion is gaining ground, that this body acts in the manner of a ferment. It is urged in favour of this view, that a ferment being an azotized substance in a state of putrefactive alteration, the body in question must find, in the decomposing organic compounds with which impure blood is charged, precisely the materials for taking on the fermenting process. The advocates for this view think that the term “zymotic” is not only the appropriate name of the whole of this class of diseases, but that it also declares an interesting fact connected with them. Whatever may be the truth with respect to these points, on which at present we have no positive knowledge, one thing is certain, that practically our concern is with the known causes,—the ascertained conditions. These are palpable, definite, and capable of complete removal and prevention.

Overcrowding, for example, we can prevent; the accumulation of filth in towns and houses we can prevent; the supply of light, air, and water, together with the several other appliances included in the all-comprehensive word CLEANLINESS, we can secure. To the extent to which it is in our power to do this, it is in our power to prevent epidemics.

The human family have now lived together in communities more than six thousand years, yet they have not learnt to make their habitations clean. At last we are beginning to learn the lesson. When we shall have mastered it, we shall have conquered epidemics. Our duties, then, and our hopes in this respect, I shall proceed to show.

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The principal constituents of the atmosphere maintain their equilibrium steadily over the whole surface of the globe. There is scarcely any difference in the relative proportion of its oxygen and nitrogen in the torrid zone and in the arctic regions. Whatever influence the atmosphere may have on climate must consequently depend on something adventitious to it and not in anything forming a part of it. Possibly therefore that something may be, in some degree, under human control.

The main constituents of climate are temperature and moisture, and these are the climatic conditions that exercise the greatest influence on epidemics.

Minor but still important conditions are the nature of the soil, the proportion of land that is cleared and under cultivation, the extent of forests, lakes, and rivers, the prevailing winds, the electrical state of the atmosphere, and so on.

The temperature is highest where the sun’s rays are vertical, or nearly so; where the sky is cloudless; where the day is longest; and where there is the smallest difference between the fervid noon-tide heat and the temperature of the short night.

The moisture is greatest where in addition to all the other sources of humidity there are periodical rains. In the countries subject to these rains, the entire extent of the level and low land is often covered a foot deeper with water than before the rain set in.

Elevated temperature and excessive moisture are combined in tropical countries; and they are concentrated in those parts of the tropics in which there are extensive forests having an undergrowth of luxuriant vegetation; in which the tides of the ocean penetrate deeply into the interior of the land, and mix with the waters of the rivers; and in which the rivers constantly overflow their banks and form marshes and swamps.

In tropical countries there are tracts such as these that extend in unbroken continuity hundreds of leagues. The western coast of Africa (the Bight of Benin) presents an unbroken area of upwards of 100,000 square miles, consisting of one vast alluvial and densely-wooded forest, irrigated by Atlantic tides, and intersected by numerous rivers and creeks, whose muddy banks are constantly overflowed.

In describing a tropical forest, Humboldt says, “Under the bushy, deep, green verdure of trees of stupendous height and size, there reigns constantly a kind of half daylight, a sort of obscurity, of which our forests of pines, oaks, and beech trees afford no example; forming a carpet of verdure, the dark tint of which augments the splendour of the aërial light.”

With this luxuriance of vegetation is combined a corresponding abundance of animal life. The earth and air teem with living creatures.

“The mould,” observes the same distinguished traveller, “contains the spoils of innumerable quantities of reptiles, worms, and insects. Wherever the soil is turned up we are struck with a mass of organic substances, which by turns are developed, transformed, and decomposed. Nature in these climates appear more active, more fruitful, we might say more prodigal of life.”

The air is still more alive than the land. Insects fill the lower strata of the atmosphere to the height of fifteen or twenty feet, like a condensed vapour. It is estimated that a cubic foot of air is often peopled by a million of winged insects, which contain a caustic and venomous liquid, several species being nearly two lines (1.8) long.

When two persons who have their home in these regions meet in the morning, the first questions they address to each other are, “How did you find the zancudoes during the night?” “How are we to-day for the mosquitoes?” An ancient form of Chinese politeness, showing the ancient state of that country, was—“Have you been incommoded in the night by serpents?”