_Prophylaxis._—Carefully avoid any kind of personal contact with water which may by any possibility be infected from urine or fæces. Bathing, wading, washing in, or drinking any such waters, are all dangerous. Wherever possible, water which may be infected should be boiled. If this cannot be done, sodium bisulphate tablets may be used; two of the 16-grain water-purifying tablets in a quart water-bottle full of water are efficient. Filtration through a Pasteur-Chamberland or Doulton candle is efficient, as it excludes the cercariæ. Bathing water can be rendered safe for immediate use by the addition of undiluted Army cresol in a dilution of 1 in 10,000. If the water is kept overnight, 1 in 90,000 is sufficient, as the storage of water tends to diminish infection. _Treatment._—Certain forms of antimony have been found to cure the condition, but can only be administered by a medical man. Indeed, either form of bilharziasis can be properly treated only under medical supervision. If this is not obtainable, some relief can be afforded by the use of urotropin and sedative drugs. There are many other worm diseases which afflict man, but no good object would be served by mentioning them in detail. Most of them can be diagnosed with certainty only by a medical man using the microscope, and they all require scientific treatment which is beyond the scope of the ordinary traveller. _Yellow Fever._ This disease, the “Yellow Jack” of naval historians, occurs, so far as is known, only in parts of the New World and on the West Coast of Africa. One attack usually protects permanently against a second. _Cause._—Quite recently the organism has apparently been discovered. It is a corkscrew-shaped parasite which exists in the blood, and which is allied to, though not identical with, the parasites of relapsing fever and tick fever. The infection is conveyed from one patient to another by the mosquito _Stegomyia fasciata_, which is a black and white insect, commonly known from its striped legs as the tiger mosquito. Incubation period two to five days. _Symptoms._—The onset of the disease is very sudden, the highest temperature being reached almost at once; then follows a period of remission or calm, the pulse becomes abnormally slow, and this stage is usually either succeeded by convalescence, or the symptoms become worse and the patient dies. Some of the symptoms much resemble malaria, but the rapidity of the onset, severe pain in the forehead, eyes, and loins, the early scantiness of the urine, the marked jaundice, the bright eyes, the narrow red tongue, and the absence of pain about the spleen are fairly characteristic.