Malaria.--The most important of these diseases is malaria in its various forms, and known under various names--chills and fever, autumnal fever, etc. This disease, so common almost everywhere, has been studied by physicians and scientists for a long time, and many have been the causes assigned to it. At one time it was thought to be the result of the growth of a bacterium, and a distinct bacillus was described as producing it. It has finally been shown, however, to be caused by a microscopic organism belonging to the group of unicellular animals, and somewhat closely related to the well-known amoeba. This organism is shown in Fig. 34. The whole history of the malarial organism is not yet known. The following statements comprise the most important facts known in regard to it, and its relation to the disease in man. Undoubtedly the malarial germ has some home outside the human body, but it is not yet very definitely known what this external home is; nor do we know from what source the human parasite is derived. It appears probable that water serves in some cases as its means of transference to man, and air in other cases. From some external source it gains access to man and finds its way into the blood. Here it attacks the red blood-corpuscles, each malarial organism making its way into a single one (Fig. 340). Here it now grows, increasing in size at the expense of the substance of the corpuscle. As it becomes larger it becomes granular, and soon shows a tendency to separate into a number of irregular masses. Finally it breaks up into many minute bodies called spores. These bodies break out of the corpuscle and for a time live a free life in the blood. After a time they make their way into other red blood-corpuscles, develop into new malarial amoeboid parasites, and repeat the growth and sporulation. This process can apparently be repeated many times without check. These organisms are thus to be regarded as parasites of the red corpuscles. It is, of course, easy to believe that an extensive parasitism and destruction of the corpuscles would be disastrous to the health of the individual, and the severity of the disease will depend upon the extent of the parasitism. Corresponding to this life history of the organism, the disease malaria is commonly characterized by a decided intermittency, periods of chill and fever alternating with periods of intermission in which these symptoms are abated. The paroxysms of the disease, characterized by the chill, occur at the time that the spores are escaping from the blood-corpuscles and floating in the blood. After they have again found their way into a blood-corpuscle the fever diminishes, and during their growth in the corpuscle until the next sporulation the individual has a rest from the more severe symptoms.