SYPHILIS is an ancient venereal or sexually transmitted disease (STD). It is caused by an unusual organism, a spiral or wavy bacterium called a spirochete. The spiral is probably caused by the flagella (endoflagella or axial fibrils) that course along the body (three are seen emerging near the "nose" of the organism at upper right). These flagella lie between the body proper (golden central cylinder) and an outer membrane (reddish surface covering). The flagella are not free as in other bacteria but they still enable the spirochete to twist and flex in a way that allows it to invade rather viscous environments. The outer membrane seems to help to cloak the treponeme from the immune system of the host.