Joint lesions are comparatively common in #scarlet fever#, and were formerly described as scarlatinal rheumatism.  The most frequent clinical type is that of a serous synovitis, occurring within a week or ten days from the onset of the fever.  Its favourite seat is in the hand and wrist, the sheaths of the extensor tendons as well as the synovial membrane of the joints being involved.  It does not tend to migrate to other joints, and rarely lasts longer than a few days.  It is probably due to the specific virus of scarlet fever.

At a later stage, especially in children and in cases in which the throat lesion is severe, an arthritis is sometimes observed that is believed to be a metastasis from the throat; it may be acute and suppurative, affect several joints, and exhibit a septicaemic or pyaemic character.

The joints of the lower extremity are especially apt to suffer; the child is seriously ill, is delirious at night, develops bed-sores over the sacrum and, it may happen that, not being expected to recover, the legs are allowed to assume contracture deformities with ankylosis or dislocation at the hip and flexion ankylosis at the knees; should the child survive, the degree of crippling may be pitiable in the extreme; prolonged orthopaedic treatment and a series of operations—­arthroplasty, osteotomies, and resections—­may be required to restore even a limited capacity of locomotion.

#Pneumococcal affections of joints#, the result of infection with the pneumococcus of Fraenkel, are being met with in increasing numbers.  The local lesion varies from a synovitis with infiltration of the synovial membrane and effusion of serum or pus, to an acute arthritis with erosion of cartilage, caries of the articular surfaces, and disorganisation of the joint.  The knee is most frequently affected, but several joints may suffer at the same time.  In most cases the joint affection makes its appearance a few days after the commencement of a pneumonia, but in a number of instances, especially among children, the lung is not specially involved, and the condition is an indication of a generalised pneumococcal infection, which may manifest itself by endocarditis, empyema, meningitis, or peritonitis, and frequently has a fatal termination.  The differential diagnosis from other forms of pyogenic infection is established by bacteriological examination of the fluid withdrawn from the joint.  The treatment is carried out on the same lines as in other pyogenic infections, considerable reliance being placed on the use of autogenous vaccines.

In #measles#, #diphtheria#, #smallpox#, #influenza#, and #dysentery#, similar joint lesions may occur.