Syphilis spirochete, also known as pale spirochete (Treponemapallidum), is a small and slender spiral microorganism, 6 to 14 μm long, 0.25 to 0.3 μm in diameter, with an average of 8 to 14 regular dense spirals. Because it is transparent and not easily stained, it is not easily detected under an ordinary microscope and can only be observed under a dark-field microscope. Syphilis spirochetes are characterized by: (1) neat spiral and fixed number; (2) strong refractive properties, brighter than other spirochetes; (3) slow and regular movement, with three types of movement: rotational movement around its long axis, or stretching the distance between its spirals to move, or bending and twisting like a snake. People are the only natural host of syphilis spirochetes. Syphilis spirochetes cannot yet be cultured in vitro to reproduce. The optimal survival temperature is 37°C, leaving the body quickly die, boiling, drying, soapy water and general disinfectants such as hydrogen peroxide, alcohol, etc. will easily kill it. The onset of syphilis is closely related to the proliferation of syphilis spirochetes in the body and the immune response they cause in the host. During sexual contact, syphilis spirochetes can be transmitted from the infected person to the sexual partner through the broken skin mucosa. After the syphilis spirochete invades the body, the incubation period is 2 to 4 weeks, during which the syphilis spirochete multiplies at the invasion site and causes the invasion site to break down through the immune response, i.e. hard chancre. Due to the strengthening of local immunity, the hard chancre can disappear on its own after 3 to 8 weeks. After the spirochetes multiply in the primary lesion, they can invade the nearby lymph nodes and then spread to other tissues and organs throughout the body via blood, resulting in syphilis rash and systemic damage such as arthritis. If left untreated, the disease may further progress to advanced stages in some patients, with cardiovascular or neurological damage, as well as dendritic damage to the skin, bone and internal organs. After syphilis infection, the body produces anti-cardiolipin antibodies and anti-syphilis spirochete antibodies, but these antibodies have no immune protective effect on the body. Early syphilis can be re-infected with syphilis after cure, while late syphilis does not re-infection, which may be related to the fact that the organism has developed cellular immunity. The source of infection is mainly early active syphilis and latent syphilis patients. The routes of transmission are: (1) sexual contact: this is the most important route. In the first year after infection with syphilis, the patient is highly infectious. With the prolongation of the disease period, the infectiousness becomes less and less; 4 years after infection, there is no infectiousness through sexual contact. (2) Mother-to-child transmission: syphilis spirochetes can infect the fetus through the placenta. It is generally believed that during the first 4 months of pregnancy, the fetus is not easily infected due to the protection of the placental cytotrophoblast; after 4 months, the syphilis spirochete is easily transmitted through the placenta due to the atrophy of the cytotrophoblast. (3) Others: In rare cases, syphilis can be transmitted through blood transfusion or some indirect ways.