Biological factors

The rate of transmission of HIV from male to female is two to three times higher than that from female to male^27,28. The Langerhans' cells of the cervix may provide a portal of entry for HIV and it has been suggested that some HIV serotypes may have higher affinity for these, and therefore to be more efficient in heterosexual transmission²⁹.

Vulval and vaginal inflammation or ulceration may facilitate entry of the virus. Sexually transmitted infections (STI) are common in many African countries, where HIV prevalence is also high^30,31,32. Inadequately treated or "silent" chlamydial and other sexually transmitted infections may act as co-factors for HIV infection and transmission^{33,34,35,36,37,38}. Syphilis rates as high as 30% have been described in antenatal women^39,40 and 4.2% of women in a population based study in the United Republic of Tanzania reported a history of genital ulceration⁴¹, which has been well established as a co-factor for HIV acquisition^42,43,44. In Zimbabwe, women reporting a history of genital ulceration and pelvic inflammatory disease were six times more likely to be HIV-positive⁴⁵. Improved STI treatment in a randomized controlled trial in the United Republic of Tanzania was shown to reduce the rate of new HIV infections⁴⁶. Other non-sexually transmitted cervical lesions, such as schistosomiasis, may also facilitate HIV infection⁴⁷. Although the evidence is still inconclusive, associations between oral and injectable contraceptive use and increased HIV risk have been reported^48,49.