It is more likely to emanate from marshy situations than from those in which the soil is dry, and hence it has been called marsh miasm. It is contained in the lower strata of the atmosphere, and is present especially between sunset and sunrise. It is a very remarkable fact that the special cause may remain for a long time latent in the system; patients sometimes do not experience the disease until many months or even years after the morbific agent has entered the body. Persons who have had periodical fever are liable during many years to relapses, without any fresh exposure to the cause. Remittent fever has been called bilious remittent, or simply bilious fever; but there is no ground for referring the pathology of this fever especially to the liver, as these names would imply.-Periodical fever maybe combined with continued fever, giving rise to a hybrid disease which of late years, in this country, has been called typho-malarial fever. The view generally held is that the special cause of periodical fever, as well as the special causes of all the essential fevers, produces its morbid manifestations by its presence and the changes which it occasions in the blood. The blood changes have been supposed to be analogous to those in fermentation, or those which are chemically called catalytic.
The name zymotic (Gr. leaven) is based on this hypothesis. The diseases which are supposed to involve fermentation or catalytic changes in the blood have been nosologically distinguished as zymotic diseases. Many cogent consider tions render it probable that the special causes of different fevers are living germs or entities, but their existence has not as yet been satisfactorily demonstrated.-2. Yellow Fever. This, although included in the class of fevers called periodical differs essentially from intermittent and remittent fever, and is a distinct species. It has doubtless its own special cause, that is, a cause peculiar to this fever. The disease prevails only in certain portions of the globe, and is rare in any but tropical or sub-tropical regions. As a rule, in these regions it is rarely prevalent in the colder months of the year. The yellow fever zone, as it is termed, is between lat. 20° S. and 40° N. The disease prevails more in the eastern than in the western hemisphere, and in certain parts of Europe and America more than in Africa. In the western world it occurs especially in the commercial towns on the Atlantic coast south of Charleston, on the gulf of Mexico, and in the West India islands.
In some seasons it prevails either as an endemic or an epidemic, and is largely destructive of human life. The mortality varies much in different seasons, the variation ranging from 10 to 75 per cent. The question as to its contagiousness has been heretofore much mooted, but at the present time comparatively few physicians regard it as communicable. The special cause, however, may be transported by means of infected vessels and merchandise, and in this way the disease is liable to be imported. Hence the disinfection of vessels coming from ports where the disease prevails, together with certain quarantine restrictions, are important. The nature of the special cause of this, as of others of the essential fevers, is unknown, but the germ theory is perhaps the most consistent with known facts relating to the history of epidemics. Facts show that the prevalence of the disease in situations where it is indigenous, and also where it has been imported, is much promoted by auxiliary causes, such as overcrowding, defective drainage or sewerage, filth, and other circumstances affecting unfavorably public health.
The special cause is destroyed by a temperature of 32° F. Irrespective of the killing effect of frost, epidemics appear to have a self-limited duration, averaging a little under 60 days. Acclimation protects against the disease, the natives of yellow fever localities, and those who have been long resident therein, being rarely attacked, although they have never experienced it; and this is one of the diseases which, as a rule, are experienced but once in a lifetime, being in this respect in striking contrast to intermittent and remittent fever. In places where the disease is indigenous, it is common for it to occur sporadically during the hot seasons; that is, cases occur, but not in a sufficient number to constitute an endemic; and when persons receive into the system the special cause in a place where the disease prevails, and going to another place experience in the latter the disease, as a rule it is not disseminated. These facts show that the special cause is not generated within the bodies of those affected.-Yellow fever generally is abrupt in its attack; that is, it is preceded by few or no premonitions as a rule. It commences with a chill, which is often not of marked intensity.
The fever varies in its intensity in different cases, as denoted by the temperature, the pulse, and other symptoms. Pain in the loins and limbs is usually a prominent symptom. The fever continues for a period ranging in different cases from a few hours to three days, when it either subsides notably or entirely ceases. In mild cases convalescence now ensues; and in a certain proportion of cases the disease is mild, and not always easily discriminated from an ephemeral fever or a febricula. In grave cases the symptoms which especially denote gravity occur after this paroxysm of fever. Among these symptoms is yellowness of the skin, or jaundice, whence the name yellow fever. This, however, does not occur in all cases, being absent in very mild attacks. It denotes a certain measure of gravity, hut is by no means a fatal omen. A much graver symptom is the vomiting of blood, or, as it is called, the black vomit. Cases very rarely end favorably when this symptom occurs. Haemorrhage in other situations, namely, the bowels, bladder, nose, eyes, and wounds which may exist on the skin, is an event denoting danger in proportion to the loss of blood.