The conference, which is held every two years and kicked off on Sunday in Washington, had avoided coming to the US in protest of a policy that dates back to 1987 that prohibited people infected with HIV from entering the country. This policy was lifted in 2010, prompting the conference's return to the US.
Under the slogan, 'Turning the Tide Together,' delegates will examine the state of the HIV/AIDS epidemic with the belief that a corner has been turned in finding a vaccine.
Recently, the Food and Drug Administration in the US approved a drug called Truvada that lowers the risk of a healthy person contracting HIV sexually from a partner.
"My colleagues and I are more optimistic to be able to find a vaccine and end the epidemic than we have been in years," Rick King of the International AIDS Vaccine Initiative told reporters ahead of the conference.
But the fight against HIV/AIDS is far from over, delegates made clear at the first day of the conference.
"The fact remains that right now, today, in the summer of 2012, 31 years after the first cases were reported, there is no excuse scientifically to say we cannot do it," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "What we need now is the political, organizational and individual will to implement what science has given us."
Before the conference got underway, about a thousand people - including many who live with HIV - marched through downtown Washington to send a message to politicians and policy makers that AIDS is still a global threat.
According to United Nations figures, 34 million people are infected with HIV around the world.
The International AIDS Conference runs through Friday.
mz/av (AFP, AP)