Answer: The sexually transmitted infection that usually fails to reveal obvious symptoms in women is syphilis.
Explanation: Syphilis is a sexually-transmitted infection, that is to say that it is a contagious disease spread by sexual activity or close contact with an infected person. The bacteria Treponema pallidum is the one that causes syphilis. Moreover, a woman that has syphilis may not feel nor see any symptoms, which means that, in some cases, syphilis' symptoms may not even be noticed.