Rheumatic serositis (serous arthritis or serositic form of rheumatism, syn. eosinophilic myositis, benign long-term fever) is an inflammation of the mucous membranes or joints, accompanied by the formation of fluid from the blood between their membranes as a result of the destruction of tissues and cells along the periphery of the areas affected by arthritis. On average, serous fluid occupies a quarter of the volume of the inflamed joint. The causative agent of the disease in humans is streptococcal infection, in animals it is Staphylococcus aureus, and less commonly, persistent herpetic infection is observed. The main route of infection is through the blood after previously suffering acute forms of gonorrhea, diphtheria, staphylococcal infection or damage to periarticular tissues or lymph nodes. Serous inflammation of the joints contributes to changes in blood circulation in the joint, compression of the vessels feeding it, therefore swelling occurs, leading to deformation of the parts of the body surrounding the joint, and subsequently to contractures. The disease develops especially in those joints where the infection initially developed, that is, most often in the peripheral and carpal-ulnar joints - of inflammatory origin. This explains the fact that the initial lesion is always aseptic in nature. The chronic course of rheumatic serositis is possible against the background of an existing skin disease (scabies, fungal infection, etc.). The joints with the capsular layer are predominantly affected, then serous-fibrous panarthritis begins with the formation of fibrous ankylosis. However, damage to both cartilage and bone tissues is possible, leading to deforming arthrosis.