yaws or frambesia, tropical infection of the skin caused by a spirochete ( Treponema pertenue ) closely related to that causing syphilis. Yaws, however, is not a sexually transmitted disease, i.e., it is not contracted by sexual contact; transmission is through ordinary contact with infected persons or their clothing and by insects. An ulcerating lesion ("mother yaw") appears at the site of contact. The second stage of the disease begins 6 to 12 weeks later, when similar ulcerating lesions appear all over the body. If the disease is not treated, the third stage develops several years later, nodular and ulcerating lesions affecting the soles of the feet ("crab yaws") and penetrating the bones with destructive changes. The first and second stages of yaws are easily treated with penicillin and other antibiotics. Yaws is rarely fatal; however, it can lead to chronic disfigurement and disability.