WASHINGTON (Reuters) – Aids transmission rates have plummeted in the United States and only 5 percent of Americans infected with the Aids virus will infect someone else in any given year, researchers reported on Tuesday.

They said prevention efforts are working, even though the number of people infected with HIV has risen.

The transmission rate has dropped 88 percent since 1984 and 33 percent since 1997, the team at Johns Hopkins University in Baltimore and the Centers for Disease Control and Prevention reported in the Journal of Acquired Immune Deficiency Syndromes.

"For every 100 persons living with HIV today, five or fewer will transmit the virus to an uninfected person in a given year," said David Holtgrave of Johns Hopkins, who led the study.

Holtgrave and colleagues based their analysis on the CDC's latest data on infections with the human immunodeficiency virus, or HIV, which causes AIDS.

In October, the CDC said 1.1 million Americans have the incurable virus and said in August that 56,300 become newly infected each year.

HIV is not especially easy to transmit.

It is passed in blood and other bodily fluids such as semen and breast milk, but not by casual contact, sneezing, kissing or on surfaces.

In the United States, it is mostly passed among men who have sex with men, although globally heterosexual transmission is more common.

Condoms are the best way to prevent transmission during sex. Male circumcision has been shown to help reduce transmission from a man to a woman, but not from man to man.

According to the study, the annual transmission rate in 1984 was 44 per 100 patients with HIV. This dropped to 6.6 per 100 people by the early 1990s and is now just under 5 per 100.

"The declines reflect the success of prevention efforts across the nation," said the CDC's Richard Wolitski.

Tests that let people know they are infected are a major factor, as is better behavior, Wolitski said in a telephone interview.

"I think it's really the result of the combination — HIV prevention efforts that include HIV testing, prevention programs for people who are living with HIV and those who are at risk for HIV as well as the effects of HIV treatment that have prolonged the lives of so many people living with HIV," he said.

"However, despite this success, we cannot forget that new HIV infections are increasing among gay and bisexual men and that African-Americans and Hispanics continue to experience disproportionate and unacceptably high rates of HIV and AIDS. The fight against HIV is far from over."