4. There is a general consensus that one attack of dengue does not protect against a second attack. De Brim in Smyrna states that "there is no acquired immunity and that frequently the same individual is attacked twice during the same epidemic; some individuals even appear to have a predisposition, and have one or two attacks in each epidemic." According to von During, on the other hand, reinfections were of rare occurrence in Constantinople, but frequent, it is alleged, in Ismailia (Sandwith) during 1883.
At the same time there cannot be the slightest doubt that dengue confers immunity for a long time in the vast majority of persons. There is an analogy here to influenza, in which the frequent relapses and recurrences gave the impression of fresh infections, and gave rise to the erroneous teaching regarding the increased predisposition after one attack. (Compare p. 577.)
Mordtmann speaks of numerous second attacks and of cases in which dengue even occurs for the third time in the same individual (after three and six weeks). On the other hand, Skottowe often saw a remittent course, but never true reinfections.
5. Enormous morbidity with very slight mortality characterizes dengue just as influenza.
In St. Denis (Reunion), during 1873, 20,000 inhabitants out of 35,000 were affected, and during 1873 in Ismailia 1800 inhabitants out of 2000. In St. Thomas, during 1827, the whole number of 12,000 inhabitants are said to have been attacked; and in 1871, of the 900 men in the garrison at Aden, no less than 700 contracted the disease. In the last great epidemic of the year 1889, 90 per cent, of the inhabitants in Cairo and Ismailia contracted dengue, and in Smyrna 100,000 persons-that is, four fifths of the inhabitants-were attacked. The usual estimate of the incidence, both in the older and in the more recent epidemics, is 75 to 80 per cent. De Brun thinks many millions were affected with dengue during 1889 in Syria, Asia Minor, Palestine, Greece, and Turkey. Von During, speaking of the distribution of dengue in Constantinople in 1889, says that "it was marvelous," and that four fifths of the population were attacked.
6. The duration of the epidemic varies with the development of the disease. The more acute its form, the more rapidly does it spread, and the shorter is its course. The average duration of an epidemic is from two to five months; in the latter case the very late, straggling cases are often counted as belonging to the epidemic.
Everybody contrasts the long duration of the "contagious" dengue with the brief six to eight weeks' duration of the supposed "miasmatic" influenza. But this is correct only for pandemic influenza; the endemic forms in the year 1891-1892, as is well known, lasted in many places for several months. (Compare p. 548.) This could also be the case with dengue. The important part played by communication can, for example, be seen in that dengue required from May, 1885, to March, 1886, to spread over the numerous Fiji Islands; and also among the scattered inhabitants in Inner Samarant (1872) dengue spread but slowly.
7. In many spots in its tropical and subtropical area of distribution dengue is an endemic disease which may at any time appear autoch thonously. As we have seen above, at certain times greater epidemic disseminations occur in numerous localities of the tropical zone, epidemics either independent of one another or evidently connected by the importation of the germs, often for great distances, from one place to another. In many places the endemic germ seems to die out until again imported; it retains its vitality for decads.
8. The distribution of dengue shows an almost absolute independence of race, nationality, position, and vocation. Earliest childhood, like in influenza, appears to enjoy a relative immunity. The unhygienic, filthy quarters of the densely populated poorer areas and the harbor districts have frequently been the primary foci and principal breeding places of infection.
The clinical picture of dengue is polymorphic. Quite apart from the great contrast naturally existing between the mildest rudimentary and the most highly developed severe case, there occur also in individual epidemics, at various times and places, marked differences in the clinical picture, differences even in cardinal symptoms-e. g., of the eruption and of the articular affections. Certain influenza epidemics (compare p. 592) were also characterized by such symptomatic variations of type.
Thus some investigators have alleged that intertropical dengue shows semeiotic differences from the extratropical. But it is very likely that some localized epidemics were in earlier times erroneously thought to be dengue. To mention but one example, this was most probably the case with the so called "dengue fever" that broke out in 1888 upon the ship "Agamemnon" after its departure from Aden.
Nevertheless, dengue in the main shows a specific, exceedingly characteristic picture. We shall now describe the typical course of the fully developed affection. There are two typical stages of the disease, to which a third may be added-the stage of convalescence.