Bound by "watta satta," a cultural tradition of exchanged marriage between two families, Nuzhat (not her real name), 22, cannot disclose her H.I.V. status.

"I know well what will happen I'll be thrown out of my husband's home and my own family will never accept me either. It will also mean my brother's home will be ruined. His wife is my husband's sister and she, too, will be sent packing. In any case, where will I go?" she asked in Karachi, provincial capital of Sindh province in southeastern Pakistan.

The tradition of watta satta, which literally means "give and take," or "throwing a stone and receiving something back," describes the exchange of brides between families, in which a brother and sister from one family are married to a pair from another family, often close relatives.

About a third of all marriages in rural Pakistan take place on a watta satta basis, according to research. In some parts of the country, like Sindh province, the rate is even higher.

Nuzhat is a classic case of the feminization of H.I.V. and AIDS that seems to have taken a toll on younger women in the Asia-Pacific region, where the epidemic is being fueled primarily by the gender inequality that prevails.

Health experts estimate there are 85,000 H.I.V.-positive people in a population of over 160 million in Pakistan but around 50 percent of them are in Sindh.