Syphilis , also known as the " pox " , is a highly contagious sexually transmitted disease , caused by a bacterium , the Treponema pallidum. During the first phase of the disease , one of the most typical manifestations is the appearance of a chancre, a small hard lesion that is mainly localized around the genitals, often at the base of the penis in men and in the vulva in women. The chancre is full of bacteria and is highly contagious . The chancre is a lesion identified by its size, rounded , painless, pinkish in color, and hard . If lesions are suggestive of a chancre, a blood serology test can confirm the diagnosis, but this test, only appearing positive after 10 days of infection , may also require study under a microscope in the early stages. If confirmed , antibiotic treatment with penicillin should be put in place, a search for other sexually transmitted diseases should be conducted, and partners should also be screened and treated if necessary. Sex should of course be protected until healed .