In addition to the blood, HIV is carried in the genital fluids of both men and women. Studies show that the genital tract functions as a separate ‘compartment’ or ‘reservoir’ of HIV. That is, HIV may be detectable in the blood but not the semen or cervical and vaginal fluids, or vice versa.

Multiple factors influence the persistence of HIV in the testes and seminal tract or the female genital tract, the most important of which are drug penetration and concurrent sexually transmitted infections. As with the brain, the genital compartment contains protective barriers that do not allow certain substances to pass through.

HIV in the genital tract has important implications for sexual and mother-to-child transmission of the virus. While, on the whole, HIV suppression lowers the risk of transmission, treatment that reduces viral load below 50 copies/ml in the blood does not always do so in the semen or cervical and vaginal fluids. Thus, transmission during sex or childbirth may still occur.