Pronunciation: (sif'u-lis),[key] —n.Pathol. a chronic infectious disease, caused by a spirochete, Treponema pallidum, usually venereal in origin but often congenital, and affecting almost any organ or tissue in the body, esp. the genitals, skin, mucous membranes, aorta, brain, liver, bones, and nerves. Cf. primary syphilis, secondary syphilis, tertiary syphilis.