The rate of HIV infections diagnosed in the United States has fallen by a third over the past decade, researchers say.

After examining cases from all 50 states, the study found that the diagnosis rate fell to 16.1 per 100,000 people in 2011 from 24.1 in 2002.

Experts celebrated the findings as a hopeful sign that the Aids epidemic may be slowing in the country.

However, there was a rise in new cases of HIV among gay and bisexual men aged under 24 and over 45.

HIV is the virus that causes Aids, a disease which destroys the immune system.

The World Health Organization estimates 35 million people globally have the virus. More than 1 million people in the US are thought to be infected, with 18% unaware of their infections.

From 2002 to 2011, 493,372 people were diagnosed with HIV in the US, researchers said.

As well as an overall decline, declines were also seen in the rates for men, women, whites, blacks, Hispanics, heterosexuals, injection drug users and most age groups.

Researchers said the only group in which diagnoses increased was gay and bisexual men.

“Among men who have sex with men, unprotected risk behaviours in the presence of high prevalence and unsuppressed viral load may continue […]