<unk>  is a  spirochaete bacterium with subspecies that cause<unk> l diseases such as syphilis, bejel, pinta, and yaws. The<unk> s have a cytoplasmic and an outer membrane. Using light microscopy,<unk> s are only visible using dark field illumination. They are gram negative, but some regard them too thin to be gram stained.
Treponema pallidum
Haemophilus ducreyi
Chlamydia trachomatis
Chlamydophila trachomatis
Cryptosporidium hominis
Herpes simplex virus
Borrelia mayonii
Borrelia afzelii