Is the world on the verge of ending the AIDS epidemic and creating an AIDS-free generation, even though a cure and a vaccine are still distant hopes?

Yes, roared enthusiasts among the nearly 24,000 participants at the 19th International AIDS Conference here last week. Their hopes are based on the extraordinary scientific gains made since the conference was last held in the United States, 22 years ago, when an AIDS diagnosis was a sure death sentence.

Among those gains: antiretroviral drug combinations for women to prevent infection of their newborns; drugs to treat and prevent infection with H.I.V., the virus that causes AIDS, among adults; and evidence that voluntary male circumcision can reduce the risk of female-to-male transmission by 50 to 60 percent.

Today, H.I.V. has become a chronic disease that, if treated appropriately, can be held at bay in a newly infected young adult for decades - if the patient adheres to the rigid daily drug regimen.

Michel Sidibe’, the executive director of the United Nations AIDS agency, said that the opportunity to end AIDS will “evaporate” if governments do not show greater political will and increase investments to make gains available to millions more people.

“All that can stop us now is indecision and lack of courage,” he said.

Ending the AIDS epidemic is likely to be far more complicated than ending most other epidemics. The AIDS conferences assemble scientists and health workers from a wide range of disciplines, infected people, activists, protesters, journalists, elected and appointed officials, pharmaceutical representatives and others. The meetings, in part, boost morale and raise the aspirations of thousands of people fighting AIDS, a laudable goal.

But since they began, in 1985, they have become more like conventions than scientific meetings. Rhetoric is plentiful, and separating it from fact is sometimes a challenge. Activists disrupt scientific presentations and news conferences with loud demonstrations, creating a circuslike atmosphere. Lobbying for more AIDS money is a given.

Dr. Richard Horton, the editor of The Lancet, the medical journal that arguably focuses most on global health, said of the conference’s new sloganeering about turning back AIDS, “It’s a marketing strategy.” He added, “It’s one that could backfire.”

One obstacle is a failure to clearly define the epidemic or what it means to have an AIDS-free generation. While many speakers repeatedly admonished that AIDS policy must be based on scientific evidence, they missed this point. The importance of precise definitions for such hopeful words extends far beyond semantics.

Definitions of terms like these may help determine how many billions of dollars the world devotes to the battle against AIDS and how many millions of lives will be extended. A failure to meet ill-defined goals could lead to public misunderstandings that limit investments and the number of people who have access to the lifesaving antiretroviral drugs in the future.

And, as Sharonann Lynch, an AIDS policy adviser for Doctors Without Borders, said in an interview, “We do have to get concrete in terms of what it means, because otherwise we are not going to be able to hold governments accountable” for how they spend taxpayers’ money to achieve specified goals.

To begin with, defining the word “epidemic” is difficult. The term is flexible and subjective, and can mean different things to different experts. Even more elusive is determining what constitutes the end of an epidemic like this one.

AIDS makes people deathly ill from problems like severe weight loss, swollen lymph nodes and a loss of critical immune cells that increases a person’s vulnerability to myriad other infections. But H.I.V. infection can remain silent for several years before causing symptoms.