Antiretroviral drugs. (Amy Sancetta/AP)

More than 700,000 of the 1.1 million people living with HIV in the United States are undiagnosed or not receiving care, a population responsible for 91.5 percent of the transmissions of the infection in 2009, researchers reported Monday.

Diagnosing even some of those people, starting them on antiretroviral drugs and keeping them in treatment could have a large and immediate impact on transmission of the virus, according to the paper, published in JAMA Internal Medicine.

"If all the people with HIV who either don't know they have the virus or are not receiving HIV clinical services were receiving care and treatment, we could expect a 90 percent reduction in new HIV infections in the United States," Jonathan Mermin, director of the National Center for HIV/AIDS, Viral Hepatitis, STD and TB Prevention at the Centers for Disease Control and Prevention, and one of the authors of the study, wrote in an e-mail. "That is a goal worth striving for."

There were about 45,000 HIV transmissions in 2009, the researchers said.

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They estimated that more than 207,000 people were infected with HIV but undiagnosed in 2009, the year they studied. Another 519,000 had received a diagnosis and perhaps some treatment, but were not retained in care, according to the study. People who typically fall out of care may have mental health or substance abuse problems, become incarcerated, lack the money for continued treatment or have difficulty accessing continued treatment, Mermin said.

"At times, there are obstacles to accessing care in the United States to HIV," he said in an interview. "Many of these people were initially engaged, or fell out of care."

The remainder of people with HIV fell into three other categories: more than 47,000 who were receiving continued care but not prescribed anti-retroviral therapy; 83,000 who were receiving the drug but not virally suppressed and 291,000 whose viral load was suppressed by treatment.

Men comprise 75.7 percent of the people living with HIV, the researchers estimated, and male-to-male sexual contact is responsible for 58 percent of the new transmissions, the study showed. Another 9.5 percent involved men who had sex with other men and used injection drugs. Injection drug use alone by men (11.2 percent) and women (6.3 percent) and heterosexual sex (7.8 percent for men, 7.1 percent for women) made up the rest of the transmission.

Given the findings, Mermin said even people who don't suspect any reason to be infected with HIV should have at least one test in their lifetimes.