A study by the Markinor polling firm and the University of South Africa shows a rapid increase in HIV infections in professional people and those with full-time employment - populations that are both key to South Africa's hopes to spur economic development.

The study, designed to reflect the country's racial and economic demographics, examined 3,500 South Africans from 2002-2005.

Overall, South Africa has a prevalence rate of 11%, or 5.5 million HIV+ people.

People with full-time jobs - about 50% of South African workers - saw HIV prevalence rise 36%, from 14.4% in 2002 to 19.2% in 2005.

Unemployed people saw a bigger percentage jump in HIV prevalence, but remained lower in terms of actual prevalence rates - with just 18.4% in 2005 compared with 11% in 2002.

Alarmingly, HIV infection is growing most quickly in those aged 30-34, threatening people just as their careers take off.

Overall, the richest third of South Africa's population still has a lower estimated HIV prevalence than the economically poorest third, at 8.5% compared to 23.4%.

But new infections are increasing most rapidly among the rich, rising by 39% between 2002-2005 against only 14% for their economically poorest compatriots.

Researchers said there were many possible factors behind the spread of HIV among upper levels of society, including confused government messages about HIV/AIDS, greater disposable income and leisure, and general apathy about safe sex practices.