Mycoplasma genitalium: a type of bacteria that can cause a sexually transmitted disease (STD), also known as a sexually transmitted infection (STI). It is estimated that up to 1% of all adults may be infected with M. genitalium. The infection does not always cause symptoms, but when symptoms do occur, they resemble those of other STDs like chlamydia and gonorrhea. In men, a burning sensation when urinating and discharge from the penis are the most common symptoms. Most infected women do not have symptoms, but some may develop vaginal discharge, abnormal bleeding (such as bleeding between periods or after sex), and pelvic pain.