In Australia, HIV infections have jumped eight percent last year and 50 percent in the past decade, say sources.
An annual report of HIV, hepatitis and sexually transmitted infections in Australia showed there were 1,137 new cases of the human immunodeficiency virus in 2011 -- an 8.2 percent increase from 2010.
The University of New South Wales, which collects the data through its Kirby Institute for infection and immunity, said new cases had increased 50 percent in the past decade.
David Wilson, head of surveillance at the Kirby Institute, said changes to testing trends among men who had sex with men -- the most affected group -- could be behind some of the marked increase.
But John de Wit, from the National Centre in HIV Social Research, warned that unprotected casual anal sex among gay men, a key risk factor, was on the increase.
One in three gay men surveyed in 2011 who reported having casual sex in the past six months said they had done so without protection, de Wit said.
"It remains critical to continue to invest in effective programmes and campaigns to reinforce condom use and inform men of the appropriate use of other sexual risk reduction practices, such as limiting condomless sex to partners of the same HIV status," he said.
According to the Kirby Institute, between 20 and 30 percent of HIV cases in Australia go undiagnosed, allowing the virus to spread.
The Australian Federation of AIDS Organisations (AFAO) said the recent spike should be a "call to action to reduce HIV infections in Australia".
"We know the tools we need, but the political will and policy action must too be there to change the number of HIV transmissions,' said AFAO director Rob Lake.
The survey found that there had been a total of 31,645 HIV cases diagnosed in Australia since the virus first broke out in the early 1980s, and there were 24,731 people living with infection in 2011.
That equated to 115 per 100,000 Australians, compared with 150 per 100,000 in Britain and 456 per 100,000 in the United States.
There are currently 34 million people living with HIV worldwide, the large majority (23.5 million) of whom are in sub-Saharan Africa.
Another 4.2 million live in South and Southeast Asia, while Oceania -- Australia's region -- has 53,000 cases.