It is not clear where yeast comes from or how people become infected. What we do know is that fungal infections are not sexually transmitted, because those who are not sexually active can contract fungal infections of the genital area—although, as noted earlier, people who are sexually active seem to get them more frequently. We also know that the bacterium normally found in the vagina, Lactobacillus, offers women protection from yeast infections, because when the relative population size of this bacterium is disturbed yeast infection is more likely. And we know that treating the sexual partners of someone who has a yeast infection does not help the person with the infection.
Women who have sex only with women may pass yeast back and forth through the use of sex toys, although there is no scientific evidence for this theory. It has also been suggested that yeast colonization occurs in the gastrointestinal tract in most people and that reinfection of the genital area occurs from this source. However, studies have produced conflicting results: some women with recurrent vaginal yeast infections do not appear to have yeast colonization in the gastrointestinal tract, and treatment of gastrointestinal yeast has not affected the recurrence rate of vaginal yeast infections.