(CNN) -- Advocates for sex workers in South Africa have warned that this summer's World Cup could be a public health disaster.

With up to half a million football fans expected to visit South Africa for the World Cup, and up to half of South Africa's prostitutes carrying the HIV virus, there have been calls for the country to decriminalize prostitution to help tackle the spread of HIV.

Eric Harper, director of the Cape Town-based Sex Worker Education and Advocacy Taskforce (SWEAT), told CNN that the World Cup would inevitably lead to a demand for sex workers.

"And where there's demand there will be a supply," Harper told CNN. "It could be a potential recipe for disaster both for the clients and the sex workers," he added.

Harper told CNN that while there are no accurate figures for the number of sex workers in South Africa, his organization believes there are 3,000 in Cape Town alone.

The UN estimates that in South Africa 5.7 million people are HIV positive, more than in any other country. A 2005 University of Michigan study found that 46 percent of female sex workers in Johannesburg had HIV.

Those figures, coupled with the expected influx of visitors for the World Cup, are causing concern for some South Africans.

Harper said HIV is just one of the dangers faced by South Africa's sex workers. As well as the risk of contracting other STIs, there is the chance of unwanted pregnancies and the ever-present threat of violence and rape.

Prostitution is illegal in South Africa, but the law is currently being reviewed as part of a larger assessment of all sexual offenses. Harper believes decriminalizing prostitution can help control the spread of HIV.

"Throughout the world people have acknowledged that if you want to reduce HIV you need to be able to engage the population and address human rights concerns," he told CNN.

"One of the key messages is that decriminalization is a way of accessing sex workers."

Harper argued that criminalization drives prostitution underground. He said that removing the threat of prosecution would make it easier to provide sex workers with condoms and make it easier for sex workers to turn down clients who refuse to use condoms.

Any possible change to the legal status of prostitution remains some way off, with South Africa's Law Reform Commission expected to make its recommendations to the Minister of Justice in 2011. Meanwhile, Harper would like to see prostitution decriminalized at least for the duration of the World Cup.

It's an idea that was also put forward in 2007 by police commissioner Jackie Selebi, now suspended on corruption charges. But Dellene Clark, of the Law Reform Commission, told CNN that the government was not considering a temporary amnesty and that it would be "nigh on impossible" to rush the legislation through parliament in time for the tournament.

Julian Seedat of the South African National AIDS Council, which advises the government on HIV and AIDS, is also expecting an increase in prostitution during the World Cup, but he is more optimistic about the health implications.

"I don't think the World Cup will necessarily bring an increased risk of the spread of HIV," he told CNN.

"Over the years there has been an incredible amount of education and awareness work done among sex workers. Years ago the high-risk groups were thought to be homosexuals and sex workers, but there has been such a focus on education for these groups that their behavior has really changed. It's quite the norm for a commercial sex worker to have a bag full of condoms."

Seedat said all public health centers in South Africa offer free voluntary counseling and HIV testing, and that organizations like SWEAT had helped educate sex workers about the importance of using condoms and being tested if they have practiced unsafe sex.

"People in the sex work sector make sure that they're protected, that they're tested and that they know their [HIV] status," he said.

Harper said most sex workers do practice safe sex, but many clients don't want to use a condom. He added that as long as prostitution remains illegal, protecting sex workers and their clients during the World Cup would be problematic.

He told CNN, "We have to make condoms freely available and we have to make it possible for sex workers to report human rights violations like child prostitution and people trafficking."