The notorious prevalence of malaria on the frontier is due to the introduction of the plasmodium into a region swarming with mosquitoes, where there are few window-screens or two-story houses. No known race has any real immunity against malaria. The negro and other colored races, it is true, are far less susceptible; but this we now know applies only to adults, as the studies of Koch in Africa showed that a large percentage of negro children had the plasmodium in their blood. No small percentage of them die of malaria, but those who recover acquire a certain degree of immunity. Possibly they may be able to acquire this immunity more easily and with less fatality than the white race, but this is the extent of their superiority in this regard. The negro races probably represent the survivors of primitive men, who were too unenterprising to get away from the tropics, and have had to adjust themselves as best they might. The serious injury wrought in the body by malaria is a household word, and a matter of painfully familiar experience. Scarcely an organ in the body escapes damage, though this may not be discovered till long after the "fever-and-ague" has been recovered from. As the parasite breeds in the red cells of the blood, naturally its first effect is to destroy huge numbers of these, producing the typical malarial _anaemia_, or bloodlessness. Instead of 5,000,000 to the cubic centimetre of blood the red cells may be reduced to 2,000,000 or even 1,500,000. The breaking down of these red cells throws their pigment or coloring-matter afloat in the blood; and soaking through all the tissues of the body, this turns a greenish-yellow and gives the well-known sallow skin and yellowish whites of the eyes of swamp-dwellers and "river-rats." The broken-down scraps of the red blood-cells, together with the toxins of the parasite, are carried to the liver and spleen to be burned up or purified in such quantities that both become congested and diseased, causing the familiar "biliousness," so characteristic of malaria. The spleen often becomes so enormously enlarged that it can be readily felt with the hand in the left side below the ribs, so that it is not only relied upon as a sign of malaria in doubtful cases, but has even received the popular name of the "ague-cake" in malarious districts. So full is the blood of the parasites, that they may actually choke up the tiny blood-vessels and capillaries in various organs, so as to block the circulation and cause serious and even fatal congestions. Obstructions of this sort may occur in the brain, the liver, the coats of the stomach, or intestines, and the kidneys; and they are the chief cause of the deadly "congestive chills," or pernicious malarial paroxysms, which we have alluded to.