**3. The pathogens**

Bacteria causing STDs have similar features related to their biology and mechanisms of infection. C. trachomatis (CT) is a Gram-negative obligate intracellular bacterium discovered in 1907, humans are its exclusive natural host; serovars associated with sexually transmitted infection are D–K. Chlamydiae are obligate intracellular bacteria with two development forms, the infectious elementary body (EB) and the active division body that is not infectious, reticulate body (RB). Once a cell is infected, the EBs can differentiate to RBs, and after a cycle of around 2 days, more EBs can be released by lysis of the host cell or by the active release of inclusions [4–7]. Neisseria gonorrhea (Ng) is also a Gram-negative bacterium obligated human pathogen described for the first time by Albert Neisser on 1879. CDC provides data on reported gonorrhea morbidity since the 1940s. Ng has evolved mechanisms for evading innate immunity and suppressing adaptive immune responses, like CT [8–10].

Syphilis pathogen is a spirochaete, Treponema pallidum (Tp); analysis based on the mutation rates of this pathogen suggests that venereal syphilis diverged several thousand years ago from Africa; this contradicts the Columbian hypothesis, in which the idea was that the shipmates of Christopher Columbus "Cristóbal Colón" brought the newly evolved venereal disease from the New World into Western Europe in the late fifteenth century; again like CT and Ng, Tp is an obligate human pathogen [11, 12].

Even the title of this chapter refers to bacterial STDs, Trichomonas vaginalis (Tv) deserved to be included since it is the most prevalent curable STD globally. Tv is a

flagellated protozoan parasite of the human genital tract. Tv has no cystic stage in its life cycle, four anterior flagella provide the parasite its characteristic motility, and its single posterior flagellum assists the motility of extracellular nutrients toward the cytosome of the cell [13].
