1. Malarial fever is caused by a small animal parasite which lives chiefly in the blood of patients attacked by it. 2. Under ordinary conditions in nature it can be conveyed only from one person to another by the bite of a mosquito which has previously sucked the blood of an infected person. 3. The parasite undergoes a series of changes in the mosquito’s body and eventually finds its way to the salivary glands of the insect whence it is injected through the mosquito’s proboscis into another victim. 4. There are only certain species of mosquitoes, belonging to the family Anophelinæ (from a Greek word signifying “harmful”), which can carry malaria, and it is only the females of these species which are affected as the males are not blood-suckers. Anopheline mosquitoes breed in shallow puddles and in almost all collections of stagnant or gently flowing water. It is therefore very dangerous to pitch camps close to stagnant pools, sluggish reed-grown streams or marshy places. 5. In the tropics one of the chief reservoirs of infection is the native, and more especially the native child, who frequently harbours the malarial parasite in the form which is adapted for life and reproduction in the mosquito and hence is a distinct source of danger. It is therefore inadvisable to camp in the vicinity of native villages or to spend a night in the neighbourhood of native habitations unless efficiently protected from the bites of mosquitoes. _Symptoms._—Malarial fever presents itself under two chief forms, though it should be noted that malaria is one of the most protean of all diseases and may simulate any malady. (1) _Intermittent_ fever. In this disease the temperature may rise high, but returns each day to normal or lower; hence there is, after each attack, a period of complete freedom from fever. An intermittent fever or ague is usually less serious than a remittent fever, but it is harder to cure in the long run. (2) _Remittent_ fever. In this the temperature, though it varies, keeps constantly above the normal, and the higher the fever, and the slighter the difference between the extremes of temperature, the more serious is the condition of the patient. Where the temperature is remittent, and appears to be unaffected by quinine, the disease is probably not malarial, but may be a case of enteric fever, and should be treated as advised below under that heading.