The form of the epidemic prevalent in any particular district is dependent on the physical characters of the immediate neighbourhood. Thus intermittents prevail chiefly in marshy and swampy districts: remittents also chiefly there, though not exclusively; while in other localities other forms arise approximating to the continued type of temperate climates.
For the most part these epidemics are strictly endemic, and are confined to the particular regions in which they are engendered. They never pass the limit of the equatorial or tropical zone. Yellow Fever, one of the most common and destructive of these diseases, is still more restricted in its range, being confined within a definite line determined by temperature. It is incapable of existing where the average range of the thermometer is greater than from 76° to 86° of Fahrenheit, or where the temperature varies more than from 5° to 10° night and day. Extreme heat and moderate cold immediately stop it; nay, even the prevalence of a cold wind for a few hours only.
In other instances these epidemics pass beyond the regions in which they are produced, and sometimes extend to all the other quarters of the globe. The Black Death, the range of which we have seen, was engendered in China; the Cholera of our own day, generated in the delta of the Ganges, the great source and centre of Indian epidemics, ravaged that country long before it directed its course to Europe.
When these tropical epidemics advance into more temperate climes, they lay aside nothing of their nature; they lose but little of their power. Wherever they go they decimate the populations which they attack.
One remarkable peculiarity of some of these epidemics is, that natives of the region in which they prevail are for the most part unsusceptible to them. This is true however only of particular forms of pestilence. Some of them acknowledge no acclimation. Cholera, for example, attacks equally natives and new comers. On the other hand, yellow fever rarely attacks the natives who reside permanently within its zone. Its chief victims are strangers who have recently arrived within its sphere, particularly the inhabitants of northern climates. The susceptibility to its influence appears to be strictly proportionate to the degree of northern latitude from which the stranger has arrived, and the shortness of the interval that has passed since he left the European for the Equatorial regions.
We see something of the same kind in the wide-spread epidemics of our own country. During the prevalence of Cholera it was observed over and over again, that persons coming directly from the pure air of the country into the infected part of a town, were seized with the disease. The explanation is not obvious. It would seem, however, to be connected with the suddenness of the shock on the system. Priestley found, that after shutting up a mouse in a given quantity of air a considerable time, it seemed to be weak, and to be slowly dying. If at this period he put a fresh mouse into the same air, it instantly died. It seems as if the system can bear a pestiferous atmosphere better when gradually than when suddenly exposed to it.
I do not know that I can give a more vivid picture of a tropical epidemic than that which is afforded by the outbreak of Cholera in the 86th regiment at Kurrachee in June, 1846.
On this occasion the atmosphere was very peculiar,—damp, hot, stagnant, and oppressive. Not a breath of air was stirring. A few isolated cases of cholera had occurred for some days. The utmost alarm was excited in the minds of experienced persons, who felt certain that an epidemic was at hand. Their fears were too fully realized. On the night of the 15th, upwards of 40 men were seized with cholera in its severest form; in two days more 256 were attacked, of whom 131 were already dead.
“The floors of the hospital,” says Dr Thom, the surgeon of the regiment, “were literally strewed with the livid bodies of men labouring under the pangs of premature dissolution. Many were brought in with the cold and clammy damp of death; as if sudden obstruction of every vital function had taken place, and the fountains of life had been arrested by an invisible but instantaneous shock. It was indeed a sight never to be forgotten, to behold the powerful frames of the finest men of a fine corps, who had that morning been in apparent good health, and most of them on the evening parade, as if at once stricken down, and striving, with the last efforts of gigantic strength, to resist a death-call that would not be refused.”
In describing a river on the west coast of Africa, Dr Daniell says—“When I visited it, I found two vessels moored a short distance from its mouth, one of which within the space of five months had buried two entire crews, a solitary person alone surviving. The other, which had arrived at a much later period, had been similarly deprived of one-half of its men, and the remainder were in such a debilitated condition as to be incapable of undertaking any active or laborious duty. Immediately before, another vessel had sailed from this port in such a deplorable state as to be solely dependent on the aid of Kroomen to perform the voyage.”