BOSTON, Aug 12 (Reuters) - Married women in India who are physically and sexually abused by their husbands are four times more likely to become infected with the AIDS virus than married women who were not abused, a Harvard study said on Tuesday.

India has the world's third-largest number of people infected with HIV at 2.47 million cases, according to the latest figures, but health workers say the number is rising rapidly and spreading to new population groups.

Researchers at the Harvard School of Public Health said they confirmed earlier studies suggesting that men who engage in extramarital sex, including unprotected sex with prostitutes, are the main source of HIV for Indian women.

More than 95 percent of HIV-positive married Indian women report being monogamous, the Harvard researchers said, adding that the extramarital sex by their husbands explains why so many victims of abuse are becoming infected.

"A women who is abused by her husband is truly placed in a situation of 'double jeopardy' regarding HIV infection," said Jay Silverman, lead author of the study published in the Journal of the American Medical Association.

"His sexual behavior outside of the marriage makes it more likely he is infected with the virus, and his abusive behavior inside the marriage leaves her with little control over sex or sexual protection," said Silverman, an associate professor at the Harvard School of Public Health.

The study examined data from India's National Family Health Survey between 2005 and 2006, with a sample that included 28,139 married women.

More than a third reported experiencing physical violence with or without sexual violence from their husbands. Of these, 28 percent experienced physical violence alone, while 7.7 percent suffered both physical and sexual abuse.

The risk for HIV infection among those who experienced both physical and sexual violence from their husbands was increased by a magnitude 3.9 over the infection risk for women who were not abused, the researchers said.

(Reporting by Jason Szep, editing by Philip Barbara)

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