A pale microbe
Treponema pallidum1 is the agent of syphilis2. Treponema pallidum can be visualized under a darkfield microscope without any staining, as a pale whitish bacterium. Due to its typical spiral shape, it can be diagnosed with high specificity. However, modern diagnostic tools such as PCR and serology have a better sensitivity for the diagnosis of syphilis.
Same family, but different mode of transmission
Treponema pallidum is a bacterium of the spirochete3 family, like the agent of Lyme disease (Borrelia) and the agent of leptospirosis (Leptospira). This shows that bacteria of the same family and similar morphology (spirochetes) can be transmitted in very different ways (inter-human by sexual transmission, by infected ticks or by exposure to rodent urines, e.g. in stagnant water bodies).
Syphilis is under-diagnosed because of the relatively few symptoms in the initial phase of the infection, generally limited to a painless ulceration5 of the genital mucosa (vulva, vagina, penis, …). In a later phase, skin lesions may appear and may often lead to a diagnosis. Fortunately, this bacterium is very sensitive to penicillin6 and its derivatives.