Biological differences primarily heighten the risk for cisgender women and receptive partners:
: Receptive partners are at greater risk because of the potential for mucosal tears during intercourse. In post-menopausal women, reduced estrogen leads to thinner vaginal mucosa, further increasing the likelihood of trauma and infection. The Vulnerability of Women and Men to HIV
: Women are estimated to be twice as likely as men to acquire HIV per heterosexual act. This is due to a larger surface area of mucous membrane exposed and the fact that semen typically has a higher viral concentration than vaginal fluids. The Vulnerability of Women and Men to HIV