Syphilis is a bacterial infection caused by the Treponema pallidum spirochete (a spirochete is a type of bacterium that is thin, long, and coiled in shape). Called the great pretender or great imitator, syphilis has a number of signs and symptoms that may mimic those of other conditions.

Syphilis is a sexually transmitted disease/sexually transmitted infection that most commonly occurs in people aged twenty to twenty-nine. Women aged twenty to twenty-four and men aged thirty-five to thirty-nine are the most likely groups to be diagnosed with syphilis. In the early years of the twenty-first century, the majority of syphilis cases have occurred in men who have sex with men.

T. pallidum uses minor cuts or abrasions to enter the body, and the infection is typically contracted by direct contact with a syphilis sore, which is called a chancre. Infection can occur with oral, vaginal, or anal sex. In addition, women who are pregnant can transmit the disease to their fetuses (called congenital syphilis). Sores are most commonly found on the external sex organs, vagina, rectum, or anus; they can occur, however, in other places (e.g., in the mouth, on the lips).

Less likely means of transmission include transfusions of infected blood, direct intimate contact with an infected partner's chancre (e.g., through kissing), or a transfer to a health-care provider during an examination or procedure. Transmission via blood transfusion is extremely unlikely because the spirochetes cannot survive long in stored blood and the blood supply is screened for syphilis. Syphilis is not spread through casual contact (e.g., commodes, pools, clothing, kitchen utensils), likely because T. pallidum is highly sensitive to light, air, and temperature fluctuations.

Infections of syphilis may progress through four stages. The infection may be spread during the primary, secondary, and early latent stages as well as from a pregnant woman to a fetus. In the primary stage the infection is usually evidenced by one sore, although there may be more than one, at the site where syphilis entered the person's body. Most often the point of entry is the penis, vagina, or vulva, but it could be another spot (e.g., lips, tongue, cervix). Typically, there is an average of twentyone days (range ten to ninety days) between infection and evidence of a sore.

Chancres are normally small, hard, painless, and round. Chancres are usually present for three to six weeks and then heal. In approximately two-thirds of cases, lymph glands in the area will be swollen. Because chancres are often small, painless, and inside the body, they can easily be overlooked. Without satisfactory treatment, however, syphilis continues into the secondary stage. In the secondary stage, common symptoms are rashes on the skin and lesions in mucous membranes. This stage usually begins with a skin rash, often one without itching, which may appear red or reddish brown in color. The rash typically emerges two to ten weeks after the chancre, following or during the healing of this sore. Although the rash may appear on one or more places on the body, frequently, such rashes appear on the palms of the hands and soles of the feet. Rashes may be light, challenging to see, and mimic those associated with other conditions. Other symptoms may occur during this stage of syphilis, including sore throat, fever, fatigue, aches, hair or weight loss, swollen lymph glands, and headaches. Regardless of whether treatment is administered, these symptoms will fade; without satisfactory treatment, however, disease progression may continue.

A third stage of the disease is the latent stage, which begins with the end of the symptoms of secondary syphilis. Early in the latent stage an individual may have no symptoms; however, one can infect others. When in late latent syphilis the risk of infecting others diminishes; without treatment, however, progression to the tertiary stage, a relapse into secondary-stage symptoms,