Like many tissues, the vagina has a natural biome, a flora and fauna of microscopic organisms. The vagina is an interface between the host and the environment. Its surface is covered by a protective epithelium where bacteria and other microorganisms grow. The ectocervix (that's the vaginal part of the cervix) is not sterile, but the endocervix (that's the canal of the cervix) and the upper genital tract are assumed to be sterile in healthy women. So, the cervix is a gatekeeper to protect the upper genital tract (ovaries and fallopian tubes) from microbes. 