In the nearly 30 years that the AIDS epidemic has raged, there has never been a more hopeful day than this.

Three striking developments took place Tuesday: U.N. officials said new HIV cases are falling dramatically worldwide. A study showed that a daily pill already on pharmacy shelves could help prevent new infections in gay men. And the pope opened the way for the use of condoms to prevent AIDS.

“I don’t know of a day where so many pieces are beginning to align for HIV prevention and treatment, and frankly with a view to ending the epidemic,” said Mitchell Warren, head of the AIDS Vaccine Advocacy Coalition, a nonprofit group that works on HIV prevention research. “This is an incredibly opportune moment, and we have to be sure we seize it.”

President Obama said the groundbreaking research on the AIDS drug “could mark the beginning of a new era in HIV prevention.”

The U.N. report said that new cases dropped nearly 20 percent over the last decade and that 33.3 million people are living with HIV now.

“We can say with confidence and conviction that we have broken the trajectory of the AIDS pandemic,” UNAIDS Executive Director Michel Sidibe said in Geneva.

Health officials credit part of the decline to wider condom use, and on Tuesday, in a historic shift in church teachings, the Vatican said that using a condom is a lesser evil than infecting a sexual partner with HIV.

Condoms remain the best weapon against AIDS, and the new prevention pill is not the chemical equivalent. But scientists called it a true breakthrough. The pill, Gilead Sciences’ Truvada, is already used to treat people with HIV. A three-year global study found that daily doses cut the risk of infection in healthy gay and bisexual men when given with condoms, counseling and other prevention services.

The drug lowered the chances of infection by 44 percent, and by 73 percent or more among men who took their pills most faithfully. Researchers had feared the pills might give a false sense of security and make men less likely to use condoms or to limit their partners, but the opposite happened – risky sex declined.

The results are “a major advance” that can help curb the epidemic in gay men, said Dr. Kevin Fenton, AIDS prevention chief at the U.S. Centers for Disease Control and Prevention. But he warned they may not apply to people exposed to HIV through male-female sex, drug use or other ways. Studies in those groups are under way.

Because Truvada is already on the market, the CDC is rushing to develop guidelines for doctors who want to use it to prevent HIV. The CDC urged people to wait until those are ready.

As a practical matter, price could limit use. The pills cost $5,000 to $14,000 a year in the United States, but roughly $140 a year in some poor nations where they’re sold in generic form.

Whether insurers or government health programs should pay for them is one of the tough issues to be sorted out, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“This is an exciting finding,” but it “is only one study in one specific study population,” so its impact on others is unknown, he said.

His institute sponsored the study with the Bill & Melinda Gates Foundation. The New England Journal of Medicine published the findings online.

It is the third AIDS prevention victory in about a year. In September 2009, scientists announced that a vaccine – which they are now trying to improve – protected 1 in 3 people from getting HIV in a study in Thailand. In July, research in South Africa showed that a vaginal gel spiked with an AIDS drug could nearly halve a woman’s chances of getting HIV from an infected partner.

Gay and bisexual men account for nearly half of the more than 1 million Americans living with HIV. Worldwide, more than 7,000 new infections occur each day. Only 5 percent to 10 percent of global cases involve sex between men.

“The condom is still the first line of defense” because it also prevents other sexually spread diseases and unwanted pregnancies, said the study leader, Dr. Robert M. Grant of the Gladstone Institutes, a private foundation affiliated with the University of California, San Francisco. But many men don’t or won’t use condoms all the time, so researchers have been testing other prevention tools.

AIDS drugs already are used to prevent infection in health care workers accidentally exposed to HIV, and in babies born to infected mothers. Taking these drugs before exposure to the virus may keep it from taking hold.

The strategy showed great promise in monkey studies using tenofovir (brand name Viread) and emtricitabine, or FTC (Emtriva), sold in combination as Truvada.

Gilead Sciences Inc. donated its Truvada for the study, which involved about 2,500 men at high risk of HIV infection in Peru, Ecuador, Brazil, South Africa, Thailand and the United States (San Francisco and Boston). The foreign sites were chosen because of high rates of HIV infection and diverse populations.

The men were given either Truvada or dummy pills. All had monthly visits to get HIV testing, more pills and counseling. Every six months, they were tested for other sexually spread diseases and treated as needed.

After a median follow-up of just over a year, there were 64 HIV infections among the 1,248 men on dummy pills, and only 36 among the 1,251 on Truvada.

Among men who took their pills at least half the time, the risk of infection fell by 50 percent. For those who took pills on 90 percent or more days, risk fell 73 percent. Tests of drug levels in the blood confirmed that more consistent pill-taking gave better protection.