Malaria, or MIASM. Malaria, an Italian word, is almost universally employed to designate an earth-born poison which is generated in soils the energies of which are not expended in the growth and sustenance of healthy vegetation. This emanation gives rise to certain diseases, especially the various forms of intermittent and remittent fever. During the past three hundred years some two thousand books and papers have been written upon the subject, but as yet, although much is known of malaria, it is impossible to state definitely what the morbid agent really is. Many theories have been advanced with regard to malaria. It has been said to be due to a parasite, to a germ, to some telluric origin which the French call telluric intoxication, to the toxic excretions of living organisms, plant or animal. Some say it is caused by chill, others by certain electrical conditions of the atmosphere, and others say it is due to a gas emitted from marsh water. Although malaria is so often connected with marshy districts as to lead many to suppose that a marsh must be inseparably connected with its production, yet we find that, although it is most powerful near marshes, it is also found in arid regions and in rocky districts which are in a state of disintegration. Its geographical distribution is very wide, but it is most virulent in tropical and subtropical regions. Its dependence on climatic influences is shown by the prevalence of malarial fever at certain seasons and under certain meteorological conditions; even in those districts where malaria is endemic throughout the whole of the year there is a maximum and minimum period to its virulence. It is certain that a high temperature is capable of increasing the frequency and severity of malarial fever; and that in those places where the summer temperature is from to F. the production of the poison is prevented. The influence of the rainfall varies with the character of the soil. As a general rule it may be said that the malarial poison is most virulent either when the rains set in after a long period of heat, or when the rains cease and give place to warm dry weather. The virulence of the poison is notably diminished at the height of the rains if they are very abundant, but the poison is produced in greater quantities in wet than in dry years. Wind exerts a certain influence upon malaria, for it can carry the poison from a marsh to a healthy spot, probably to the distance of three miles; wind also prevents the vertical ascent of the poison, for in calm air malaria may ascend to 700 or 1000 feet above a swamp, but should a strong wind be blowing this vertical diffusion is prevented. It is also probable that on some islands where malaria is absent, although from analogy we should expect to find it present, the wind by rapidly changing the atmosphere carries away the poison before it has time to do harm. The extent and severity of malarial diseases diminish as one ascends above sea-level. The height at which malaria is produced depends upon climatological conditions—i.e. in tropical districts one must ascend to a greater altitude to find a situation free from malaria than would be necessary in a northern climate. In the tropics an altitude of 3200 feet may be required to prevent the production of malaria, whereas in the Apennines 1500 feet only is required, and farther north malaria is not found at a greater altitude than 500 feet. In investigating the occurrence of malaria in non-mountainous regions we also find that altitude plays a not unimportant part, for even on a level plateau with basin-like depressions the deepest points are those most affected by disease, and in those exceptional cases where malaria is endemic at more or less considerable elevations the seat of the production of the poison is invariably in a valley or ravine. The older geological formations are more or less exempt in proportion to the compactness of the rock and the porosity of the soil, and accordingly the alluvial and diluvial formations are the chief seats of endemic malaria. Clay, marl, and marsh lands are most favourable to its production; a porous chalky soil is less favourable, and sandy soil least favourable to its production. Where malaria is endemic in rocky districts there is always a more or less thick layer of permeable alluvial, diluvial, or mineral detritus spread over the firm rock, and invariably a hygroscopic upper soil.
Whatever the character of the soil, a copious saturation of the ground is necessary for the production of malaria. This may be caused by atmospheric precipitations, drainage from rivers, lakes, or pools, inundations, either periodic or irregular, irrigation, and saturation of the ground with subsoil water. The amount of organic matter in the soil is certainly connected with the production of malaria, and, other circumstances being equal, the greatest amount of malaria will be found where the amount of organic matter in the soil is greatest, least where it is least. Changes in the soil indicate clearly its influence on the production of malaria, for the latter will disappear on damp or marshy soil being completely dried up. If water completely covers the soil, malaria disappears; if virgin land be reclaimed, it proves malarious until perfectly cultivated, and a neglect of once cultivated ground may also produce malaria. It has been found by experience that malaria is not produced in the centre of cities, and, although it may be prevalent in the outskirts of large towns, it does not penetrate to their centres.
Laveran discovered the parasite of malaria; Bignami, Grassi, and others insisted (1899) that the malady was mainly communicated by mosquitoes; and Major Ross made extensive researches in West Africa into the theory, singling out the Anopheles claviger as especially active. Professor Koch's investigations (1899–1900) in Italy, Java, and New Guinea prove that the true home of the germ is in the blood of the malarial patient; mosquitoes and other gnats, imbibing the germ in stinging a sufferer, introduce it into the blood of the next person stung. In New Guinea nearly all children are afflicted with malaria. Much may be done to combat the disease by special attention to children, and to mild and latent cases in adults. In Germany cases of malaria in the army, which in 1865 were 13,500, were in 1895 only 230; doubtless owing to the free use of quinine. Ross insists on the possibility (denied by Koch) of exterminating mosquitoes by surface drainage. Dr Sambon and Dr S. Low, who were commissioned by the British Government, spent the malarial season (July to October) of 1900 in the worst parts of the Roman Campagna; they found that people who only go about during the day, when these insects do not appear, and who protect themselves from them at night, when they are abroad in myriads, do not suffer from malaria; whereas the sting of a mosquito was invariably followed by the disease. See ENDEMIC and the monograph by Celli (trans. 1900).