It must be true that many cases of individual resistance to the effects of yellow-fever infection depend upon states of the system or idiosyncrasies which diminish liability to the action of the poison. In other words, their personal receptivity to it is lessened by certain constitutional states.
That this position is correctly taken is proved by the fact that many circumstances which violently disturb the system determine attacks in persons who may have for a long time enjoyed immunity from them. Anxiety, grief, fright, fatigue, or exposure to sudden wettings or cold may precipitate attacks, either by disturbing vital processes by which the system is ridding itself of the poison—so far, at least, as to prevent an accumulation great enough to occasion attacks—or by lowering powers of resistance through enfeeblement of nerve-force.
But it can be affirmed in regard to yellow-fever poison that it is not more capricious or eccentric in its behavior as an infection than that of scarlet fever. Each of these diseases may appear in a large family of unprotected persons with a degree of violence which results in death in every instance, and suddenly cease, leaving a greater or less number of the household without attacks, though equally exposed with those who have died.
One attack of yellow fever confers immunity from the disease during after life. A person who has suffered an attack is said to be acclimated or protected. Neither of these terms should be applied to those who have not suffered attacks, however long they may have withstood exposure during epidemics. It often occurs that persons who have escaped attacks through many years of renewed exposure at last succumb to the disease. On the other hand, I know of three well-authenticated instances of immunity in a sweeping epidemic of persons whose mothers had suffered attacks during the gestations which respectively resulted in their births.
While negroes are susceptible to yellow-fever infection, attacks are far less fatal than among whites.
SYMPTOMS IN MILD OR SIMPLE CASES.—Yellow fever is usually sudden in its onset. Persons are liable to be seized while pursuing their ordinary avocations, or, as often occurs, the attack may begin during the night. The initial symptoms are chilliness or cold sensations, seldom amounting to a decided rigor. Reaction is usually prompt and decided, the temperature reaching within a few hours 102° to 105° F. Yellow fever is not a disease in which it is very common to observe excessive body heat.
As the fever is established, the countenance becomes flushed and the eyes injected and glistening. Frontal headache and lumbar pain are experienced very early in the attack, and are liable to become more intense during the progress of the fever. Muscular neuralgias, especially in the lower extremities, are not uncommon.
During the early period of the attack the tongue is indifferent as a symptom. It is generally moist and free from any coating. In cases attended by much furring of the tongue careful investigation is pretty sure to disclose the fact that it has been brought about by some pre-existing state of disease.
The bowels are generally inactive, though naturally impressible to cathartic drugs. The stomach is querulous from the inception of the attack to its conclusion. Vomiting may not occur spontaneously, but it is easily provoked by repletion of the stomach with any description of ingesta or by harsh or disgusting medicines. The acts of emesis are sudden and short in duration. Bile is a very uncommon constituent of the matters ejected. Whether vomiting has occurred or not, patients nearly always express repugnance to the weight of the physician's hand over the epigastrium. In the very mildest cases it seems to excite gastric distress and a tendency to emesis. The stomach and bowels are liable to distension by flatus, sometimes to the extent of producing colicky pains. Gaseous eructations are common.
During and shortly succeeding the cold stage the urine may be somewhat increased in amount, but after the fever is established both the quantity and the specific gravity are notably lessened. Albumen seldom appears in the urine during the first twenty-four hours of an attack. In very mild cases it is altogether absent throughout.