<unk>  is a spirochaete bacterium with various subspecies that cause the diseases syphilis, bejel (also known as endemic syphilis), and yaws. It is transmitted only amongst humans. It is a helically coiled microorganism usually 6–15 μm long and 0.1–0.2 μm wide.<unk> s lack of either a tricarboxylic acid cycle or oxidative phosphorylation results in minimal metabolic activity. The<unk> s have a cytoplasmic and an outer membrane. Using light microscopy,<unk> s are visible only by using dark field illumination.<unk>  consists of three subspecies, T. p.<unk> , T. p. endemicum, and T. p. pertenue, each of which has a distinct associated disease.
Treponema pallidum
Haemophilus ducreyi
Chlamydia trachomatis
Herpes simplex virus
Mycoplasma genitalium
Veillonella parvula
Toxoplasma gondii
Cryptosporidium parvum