The International AIDS Conference, which is held every two years, opens on Sunday and runs in the Mexican capital until Friday, and coincides with a relative lull in the war against the disease. The expected turnout runs over 22,000.
The theme, ''Universal Action NOW,'' reflects an appeal to political leaders to maintain the momentum that began to build in mid-decade and has transformed access to precious antiretroviral drugs in poor countries.
A ceremonial concert late Sunday was to give the gathering the official kickoff, but workshops, seminars and other activities began several days before the start.
VIPs include former president Bill Clinton, a key figure in the campaign to slash the price of anti-HIV drugs to developing countries, which are home to 90 percent of the 33 million people with the AIDS virus.
Insiders said they did not expect any breakthrough announcement in the arena of drugs, and braced for confirmation that the quest for a vaccine and an HIV-thwarting vaginal gel was mired in setbacks.
More positively, though, evidence has emerged that male circumcision can help prevent HIV infection among men -- a finding of great significance in southern Africa, the epicenter of the pandemic.
More than 25 million people have died from AIDS and 33 million today have the virus that causes it, according to UN figures.
Thanks to a major increase in funding and cuts in the price of first-generation antiretrovirals, nearly three million needy people in developing countries have access to the lifeline drugs.
The triple ''cocktail'' rolls back the virus, thus helping to restore the immune defenses, but does not completely eradicate the pathogen.
''There has been a spectacular advance, but we are still very short of the mark,'' Jean-Francois Delfraissy, head of France's National Agency for AIDS Research (ANRS) told AFP.
''One of the tasks of the conference is to address the fact that there are three million people who now get the drugs, but another nine million who do not.''
According to UN agency UNAIDS, around 10 billion dollars was spent last year fighting AIDS in poor countries, but this was eight billion dollars short of what was needed.
Simply to maintain the current pace of drug access will require funding to rise by 50 percent by 2010. Even more will be needed to meet the goal of universal access, set for that year by the UN General Assembly.
The rising costs of keeping people alive under antiretroviral treatment -- a daily regime of pill-taking that has to be maintained for the rest of the patient's life -- is likely to revive focus on preventing the spread of the virus.
Workshops at the conference, the 17th in the series, will focus on exploring new ways, and on swapping ideas, for preventing transmission of the virus among intravenous drug users, people with multiple sex partners and other vulnerable groups.