Syphilis ulcer. Light micrograph of a section through a primary chancre (ulcer) due to syphilis. This sexually transmitted disease is caused by the Treponema pallidum bacterium. The first symptom is a chancre at the site of infection on the genitals. The chancre is a raised, reddened nodule caused by local accumulation of plasma cells and lymphocytes in the subepithelial connective tissue. This chancre may go unnoticed or may ulcerate. It eventually heals, but secondary and tertiary syphilis may follow. Syphilis is treated with penicillin antibiotic drugs.