In this disease septic microbes are deposited in the joint and spread over the surface by the synovial fluid. The result is an acute inflammation with fibrinous exudation, but generally going rapidly on to suppuration. It is remarkable that, when the joint is full of pus, there is sometimes very little structural change in the synovial membrane, the whole condition being almost confined to the blood-vessels from which an excessive exudation has occurred. The inflammation usually affects several joints simultaneously.

It is to be added that occasionally in Dysentery, Diphtheria, Scarlet fever, Erysipelas, etc., a similar acute arthritis occurs. In these diseases there is a breach of a cutaneous or mucous surface and septic or other microbes may find entrance. The affection of the joints is thus similar in its origin to Ulcerative endocarditis, which sometimes complicates these affections.