Diseases caused by other organisms than bacteria.

Although the purpose of this work is to deal primarily with the bacterial world, it would hardly be fitting to leave the subject without some reference to diseases caused by organisms which do not belong to the group of bacteria.  While most of the so-called germ diseases are caused by the bacteria which we have been studying in the previous chapters, there are some whose inciting cause is to be found among organisms belonging to other groups.  Some of these are plants of a higher organization than bacteria, but others are undoubtedly microscopic animals.  Their life habits are somewhat different from those of bacteria, and hence the course of the diseases is commonly different.  Of the diseases thus produced by microscopic animals or by higher plants, one or two are of importance enough to deserve special mention here.

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.