Syphilis is caused by a spiral bacterium (aka a spirochete) known as Treponema pallidum. The bacterium is usually acquired during sexual contact, whereupon it corkscrews its way across mucous membranes, multiplies in the blood and lymph nodes, and, if a patient is especially unlucky, gets into the central nervous system, including the brain, causing personality change, psychosis, depression, dementia, and death.
