Obviously, the picture of acute rheumatism carries upon its face a strong suggestion of its real nature and causation. The high temperature, the headache, the sweats, the fierce attack and rapid decline, the self-limited course, the tendency to spread from one joint to another, from the joints to the heart, from the heart to the lungs and the kidneys, all stamp it unmistakably as an infection, a fever. On the other hand, there are two rather important elements lacking in the infection-picture: one, that, although it does at times occur in epidemics, it is very seldom transmitted to others; the other, that one attack does not produce immunity or protect against another. The majority of experts are now practically agreed that _acute_ rheumatism, or _rheumatic fever_, is probably due to the invasion of the system by some microoerganism or germ. When, however, we come to fixing upon the particular bacillus, or micrococcus, there is a wide divergence of opinion, some six or seven different eminent investigators having each his favorite candidate for the doubtful honor. In fact, it is our inability as yet positively to identify and agree upon the causal germ that makes our knowledge of the entire subject still so regrettably vague, and renders either a definite classification or successful treatment so difficult. The attitude of the most careful and experienced physicians and broad-minded bacteriologists may be roughly summed up in the statement that acute rheumatism is probably due to some germ or germs, but that the question is still open which particular germ is at fault, and even whether the group of symptoms which we call rheumatism may not possibly be produced by a number of different organisms, acting upon a particular type of constitution or susceptibility. There is no difficulty in finding germs of all sorts, principally micrococci, in the blood, in the tissues about the joints, and on the heart-valves of patients with rheumatism, and these germs, when injected into animals, will not infrequently produce fever and inflammatory changes in the joints, roughly resembling rheumatism. But the difficulty so far has been, first, that these organisms are of several different kinds and distinct species; and second, and even more important, that almost any of the organisms of the common infectious diseases are capable at times of producing inflammation of the joints and tendons. For instance, the third commonest point of attack of the tubercle bacillus, after the lungs and the glands, is the bones and joints, as illustrated in the sadly familiar "white-swelling of the knee" and hip-joint disease. All the so-called septic organisms, which produce suppuration and blood-poisoning in wounds and surgery, may, and very frequently do, attack the joints; while nearly all the common infections, such as typhoid, scarlet fever, pneumonia, and even measles, influenza, and tonsillitis, may be followed by severe joint symptoms.