One of the most important questions that arose in connection with malaria was, In what manner does the human being become infected with malaria? On the correct solution of this problem depended the rational prophylaxis, and it was, therefore, reasonable that so much should have been done, especially since the discovery of the malarial parasite, with this view in mind.

The problem has now been solved; it will not be unprofitable, however, to consider the older views and to trace the steps whereby the solution has been reached. There were three hypotheses in regard to this. Not one of them is new, all coming to us from ancient times. Sometimes one obtained, again another. The three theories were: 1. The water theory. 2. The air theory. 3. The mosquito theory.

I. The Water Theory

Numerous data have been brought forward in support of the transmission of malaria by drinking water. Though a number of these appear at first sight rather convincing, further consideration shows that no proofs have ever been offered to show that any one became infected with malaria from drinking contaminated, water. The only experiment that would prove it with certainty would be, if malaria were produced in a person in a region absolutely free from malaria by the drinking of water from a malarial region. This experiment has been done, but with negative result. Celli had several people in the Roman Hospital San Spirito drink water for several days from the Pontine marshes and the swamps about Rome, but found no malaria develop. Brancaleone repeated this experiment in Sicily, with the same negative result. Zeri had 9 persons drink, for five to twenty days, 1.5 to 3 liters of water daily (in all, 10 to 16 liters each) from a malarial region; he had 16 persons inhale the same water in an atomized condition; to 5 persons he administered it by the rectum. None of them became infected with malaria. Salomone Marino also described similar negative results. The attempts, therefore, to produce malaria by means of water have so far been in vain.

In addition, there are still other facts which make these experiments more convincing. It is often necessary for ships' crews to take water from severely infected malarial coasts, to be used for drinking in the course of the voyage. Yet malarial infection on ships is extremely rare, presupposing, naturally, that the crew has not gone on land. Bouclin's much quoted case of the ship "Argo," which left Bona in July, 1834, and on which the soldiers who drank the water taken on board at Bona were attacked by malaria , while the sailors who drank other water remained healthy, has been the occasion of much argument. This case may now be looked on as settled, for there seems no doubt that the soldiers acquired then infection before going on board.

Valuable critical evidence has been added, and an interesting and striking fact stated by Rupert Norton. This writer showed that in a large number of American forts where malaria is endemic the absolute malarial morbidity has not been altered by the conduction to them of pure water.

Laveran was himself skeptical in relation to the water theory, yet seemed unwilling to dispose of it entirely. Kelsch and Kiener follow Colin, who, after a critical study, took a stand against the water theory; Fayrer and Manson, on the contrary, defended it.

Fayrer asserted that people who drink filtered water suffer less frequently from malaria than others, but, unfortunately, gives no figures, and does not even say if this is true, ceteris paribus.

Manson formed his conclusion on an experiment of Ross's. The latter had an Indian native drink one or two drams of water in which mosquitos had died that had previously fed on malarial blood. The mosquitos themselves were removed before the beginning of the experiment. Eleven days later the man was attacked by fever that ceased spontaneously after a few days. Ring form parasites of the second group were demonstrated in large numbers in the blood; there were no crescents and no relapse followed.

The experiment was later repeated on other individuals, but with negative results. The first positive experiment loses most of its value on account of being done in a malarial region, where infection in other ways could not be excluded.

Isolated facts apparently supporting the water theory have been collected by Laveran, and for further arguments we refer the reader to his work.

2. The Air Theory

The view that malarial parasites rise from the soil into the air and gain entrance into the organism by way of the respiratory tract is one of the oldest and most common. The name "malaria " shows that this conviction arose among the people themselves. But in science, popular instinct and public opinion carry no weight, and it is exactly the air theory that, after years of tranquillity, has fallen under sharp criticism and been forced into the background.

Several facts are opposed to the air theory which cannot be explained by it, at least in a satisfactory way.

It is, first, very striking that severely infected and almost immune places may lie in close connection with one another; that in the same city the inhabitants of one quarter may suffer from malaria and those of another not; that in the same house, rooms looking in a certain direction may be dangerous, and others not; and that an elevation of a few meters above a severely infected swampy soil is sufficient to protect from infection. Examples of this kind are very numerous. So, for instance, it is known that crews of ships lying anchored on severely infected coasts (presupposing, of course, that they do not go on land, and that the distance from the shore is not entirely too small) are seldom attacked by the disease. Pringle mentions that the forces remaining on the English squadron anchored near the island of Walcheren were spared, while those on land suffered severely from malaria. In the Madagascar campaign (1895) the ships anchored scarcely 300 meters from the pestilential shores, yet the crews remained healthy. At Rome, outside the Porta del Popolo, malaria rages; a few hundred meters from there, the Corso is not at all dangerous. The hospital San Michele a Ripa Grande is immune; a short distance from it toward San Paolo there are severe malaria foci. The inhabitants of Paolo assert that in certain houses the rooms looking toward the sea are secure, while those looking toward the country are scourges (quoted after Bignami). Similar observations are made in great numbers from all malarial regions. In Vienna we recognize that fever occurs only about the bank of the Danube (Prater, the Danube meadows, etc.).