It takes two to tango but one to infect (on the underestimation of the calculated risk for infection with HIV in sexual encounters, arising from nondisclosure of previous risk behavior or seropositivity).

The effects of nondisclosure of either HIV seropositivity or previous engagement in risk behavior on the estimation of risk in sexual intercourse is studied through the use of a probability model. Equations are utilized to derive an Underestimation Factor for three cases, which is shown to range from 2 to 20 in the case of insertive anal intercourse between an HIV+ and an HIV- man, and to 1 x 10(4) in a similar case of heterosexual insertive vaginal intercourse. This factor remains constant regardless of the number of sexual encounters between the same partners, and regardless of the protective measures as long as unawareness prevails. The discussion focuses on the implications of this formulation for education and social attitudes toward disclosure and nondisclosure in sexual encounters. It also raises the question of moral responsibility of sexual partners and some legal aspects of nondisclosure.