"Many of those who are coming have told their work or families that they are going on vacation," Manuel da Quinta, who works for UNAIDS in Geneva and has been HIV-positive for 13 years, told AFP.
"They're afraid of saying they're taking part in an AIDS conference."
Media coverage of the conference will be restricted, with participants wearing different-colored stickers to show whether they accept being filmed and photographed or not, said a member of the organizing committee.
Those who agree to speak may request that their interviews will not be broadcast in their home countries, like 32-year-old Anastasia, from Russia.
"I contracted AIDS after having unprotected sex. The guy didn't tell me (he had HIV). I fell in love him so I didn't use a condom," she said. "He was the first patient with AIDS in my town ... He was a soldier who had served in Africa."
Around 33 million people had the AIDS virus in 2007, the majority -- some 22 million -- in Sub-Saharan Africa, according to the latest UN figures.
Auguste Dokla, president of a network of AIDS patients in Togo, said that progress to raise AIDS awareness in the west African country was slow.
"We demonstrate on the streets to ask for medicines, but our meetings are set up by word of mouth," she said. "We don't have access to radio and television (to publicize our cause)."
Some 2,500 of the 22,000 expected at the first world AIDS conference in Latin America have said they are HIV-positive, but organisers believe many more may have omitted to declare they are carrying the disease.
Da Quinta, from Portugal, checks with participants to see whether they wish to speak to the media during the meeting.
"I didn't suffer too much from discrimination. It's different in Europe," he said. "But in Latin America it's always very hard due to the culture (and) the Catholic Church.
"Everything is shameful here. It's shameful to be a homosexual, a prostitute, to have AIDS."