In about 25% of women in the reproductive period, the parasites harbor in the vagina is in asymptomatic state. When the local defense is impaired—during and after menstruation, after sexual stimulation, and following illness, the pH of the vagina is raised to 5.5–6.5. At this level of pH, the trichomonads thrive. The organisms usually lie in between the rugae and produce surface inflammatory reaction when the defense is lost.