Mechai Viravaidya bounded through the hallways of a local bank one lunch hour this week, cheerily dispensing green and orange condom packets to clerks, accountants and analysts, many of them young and single.

"Condoms are a girl's best friend!" he said -- part comic, part missionary -- to the women.

"Looks like you need a green one," he deadpanned to one man, joking that the greens and oranges were different sizes.

Mechai, who is known by his first name, is Thailand's condom king, set on getting people to protect themselves against AIDS by using condoms during sexual intercourse. He has been one of the central figures in Thailand's remarkable effort, begun in earnest in 1991, to combat a disease with no cure and no vaccine that has killed more than 20 million people worldwide.

From 1991 to 2003, Thailand registered a drop in annual new infections from an estimated 143,000 to about 19,000, an achievement credited to a combination of political leadership, increased funding, public awareness campaigns and a pragmatic effort to work with prostitutes to promote condom use.

The 15th International AIDS Conference, which opens Sunday in Thailand, has recognized the country as a model, one of just a few nations that have reversed the explosion in rates of infection.

"Thailand is a leading light in the global fight against AIDS," said Hakan Bjorkman of the U. N. Development Program, who led a team that wrote a new report on Thailand's response to AIDS.

But there are fresh indications in some sectors of Thai society that the danger has not passed. U.N. officials said new infections are rising and remain high among drug users and homosexuals. They also said a growing majority of young people are not protecting themselves during sex. "Now the epidemic has evolved, and there are warning signs that Thailand may be in for a nasty surprise," Bjorkman said.

About 600,000 Thais have HIV, the virus that causes AIDS, and 53,000 of those infected die as a result of AIDS each year, according to official estimates.

About 12 percent of prostitutes are still infected, with rates highest in brothels near the Burmese border. One study of gay men in Bangkok found that 17 percent of those tested were infected. HIV is beginning to spread in the general population in southern Thailand, especially among pregnant women.

Though the number of intravenous drug users has decreased, the number of those infected has climbed to as high as 50 percent, according to official estimates. Human Rights Watch, a New York-based organization, this week charged that the government's hard-line crackdown on drug dealing has driven individual drug users underground, making treatment and prevention harder.

Most worrying, the U.N. report said, HIV/AIDS is now the leading cause of death among people ages 15 to 44 in Thailand. Only 20 percent of sexually active young people use condoms consistently, and only 15 percent of young gay men report using condoms consistently.

Thailand's financial commitment to prevention has dwindled, the U.N. agency said. Between 1997 and 2003, according to the report, the prevention budget was sliced by 62 percent, to $2 million. Now, prevention commands only 8 percent of the AIDS budget, the report said.

"There's been a loss of momentum," said former prime minister Anand Panyarachun. There is "no top-level political commitment. AIDS is still a serious problem in Thailand."

Activists for Change

Mechai has been a public face of the AIDS prevention program -- so much so that in Thailand, his name has adorned condom wrappers and become slang for the word condom in the Thai language.

He became involved with AIDS awareness shortly after Thailand's first documented AIDS case in 1984 -- a gay man who had studied in the United States, where he had become infected with HIV. While infections among gay men remained low, by 1988, rates had exploded among injected-drug users in Bangkok.

In those years, the government was largely in denial, recalled Mechai, who was then a government spokesman. Officials feared the country's image would suffer and tourism might collapse, he said. When the health minister tried to raise awareness in a speech, travel industry workers burned his effigy. The government then banned discussing AIDS education in official media, Mechai said.

But in some quarters, officials were beginning to argue for change. In 1989, a health ministry official, Wiwat Rojanapithayakorn, piloted an innovative campaign for universal condom use to drive down infections among prostitutes. The program would monitor every brothel, massage parlor, teahouse and sex worker in the province of Ratchaburi, south of Bangkok, where HIV infections were higher than in neighboring provinces.

He invited all 370 prostitutes known to be working in the province to a meeting. The rules were simple: no condom, no sex. If everyone cooperated, no one would lose business.

The women got the concept and shared the information with one another, he said. "The whole province followed the rule." Sexually transmitted infections became rare.

To enforce the rule, Wiwat relied on local health clinics, part of an extensive network set up across Thailand during the Vietnam War. Prostitutes routinely visited the clinics for health checkups.

If a man or a prostitute showed up with a sexually transmitted infection, Wiwat said, health workers could deduce that no condom had been used. The clinic would then identify the brothel, massage parlor or teahouse and issue a warning. A second infraction would lead to the establishment being closed. Everyone got the message, and there were no closings, Wiwat said.

A National Campaign

The nationwide AIDS program began in 1991 after Anand took over as prime minister. Anand later asked Mechai to brief him on the AIDS situation. "The statistics were quite scary," Anand recalled.

At the time, Mechai warned that without immediate action, 4 million Thais would be infected by 2000. He said the disease would cut an average of 25 years off a person's working life, resulting in a loss of $10,000 in earnings per person. "Multiply that by 4 million," Mechai said. "It's huge."

Mechai convinced Anand that he, not the health minister, should chair the National AIDS Prevention and Control Committee, which met several times a year.

Ministry officials, doctors, religious leaders, activists and travel industry representatives were recruited. Debate raged between treatment and prevention advocates, but prevention became a priority, Anand recalled. State-owned radio and television stations were required to broadcast 30 seconds of AIDS education every hour on the air. "One ad began with a Buddhist cremation chant," Mechai recalled. "Then came the voiceover: 'Before long, you'll be hearing more of this than the Top 10 songs. Be careful of AIDS.' "

Another ad, pitched to a country that reveres its royal family, showed the wife of the crown prince welcoming HIV-positive people into the palace. "She talked with them, touched them," he said.

Then Mechai took Wiwat's 100 percent condom-use campaign national. At the time, prostitution was at the epicenter of the outbreak. In all 75 provinces, each commercial sex establishment had to have condoms, and the no-condom, no-sex rule was enforced. The government distributed tens of millions of free condoms.

In Thailand, where prostitution is illegal, "we didn't want to make moral judgments," Mechai said. "We wanted to save lives."

The education activities were "quite powerful," said Wassamon Likhitsakulchai, the owner of the Rainbow Bar club, as young women wearing neon thong bikinis danced nearby. "After that, the girls insisted on using condoms every time. If the clients refused, they just canceled the deal."

Pragmatism guided Thailand's approach. Abstinence is not bad, but it is not adequate, said Suwit Wibulpolprasert, a Health Ministry official. "If you worked only on abstinence in Thailand, you might have 1 million more HIV infections here."

Anand's administration lasted two years, but subsequent governments carried on the commitment. By 1996, the annual AIDS control budget was $82 million, 96 percent of it financed by the Thai government, the U.N. report said.

Health experts said the government of Prime Minister Thaksin Shinawatra needed to take new initiatives to fight the rise in HIV infections. "Thailand has been in hibernation for 21/2 years," he said. "Care and treatment was going on, but public education just disappeared."

Suwit acknowledged that the government needed to address infections among drug users and young people and that the prevention budget had been reduced. But, he argued, the declining overall rates meant that the initial investment in education was still paying off.

Bjorkman, of the United Nations, said Thailand must not slip up. "If Thailand falters, the world loses a model to follow," he said. "The good work it has done will be discredited. We need Thailand as an inspiration."

A Thai waiter wears a condom on his head at Bangkok's Cabbages and Condoms restaurant, founded by Mechai Viravaidya to promote safe sex education. Mechai said the government must take new steps against AIDS.