WASHINGTON -- The head of the United Nations' AIDS program warned yesterday that India, China and Russia are "perilously close to a tipping point" that could turn their small, localized AIDS epidemics into gigantic ones capable of disrupting the whole world's response to the disease.

The situation in those three countries "bears alarming similarities to the situation we faced 20 years ago in Africa," the Belgian physician and epidemiologist Peter Piot told policymakers in Washington. It could transform "from a series of concentrated outbreaks and hot spots into a generalized explosion across the entire population."

If that happens, affecting both the global economy and international security, "no country on Earth will escape the impact," said Piot, who heads UNAIDS, a program run by U.N. agencies, the World Bank and the World Health Organization.

While many experts in recent years have publicly worried about the emerging AIDS epidemics in China and India, Piot's warning was unusually ominous and concrete.

He also drew a connection between AIDS prevention efforts in Asia and AIDS treatment efforts in Africa, which have sometimes been viewed as potential competitors for scarce resources. Specifically, he argued, the laborious, expensive and overdue effort to bring antiretroviral therapy to millions on the continent where AIDS began could unravel if Africa-scale epidemics in India and China steal the world's attention.

Piot spoke at the Woodrow Wilson International Center yesterday on the eve of World AIDS Day.

In making his remarks yesterday, he shared a podium with Randall Tobias, the director of the Bush administration's global AIDS activities. None of the three large countries Piot discussed is among the 15 "focus countries" targeted by the $15 billion President's Emergency Plan for AIDS Relief.

In emphasizing the importance of China, India and Russia, Piot said he intended no slight to the administration's AIDS effort in 12 countries in sub-Saharan Africa, Haiti and Guyana in the Americas, and Vietnam. He called "the leadership and generosity of America ... one of the most promising and heartening developments in years."

But he warned: "If the epidemic gains a foothold in even a few states or provinces in China and India, and spreads there as it has in some African countries, the global resources now available for Africa could easily diminish, perhaps even vanish. If we hope to have the resources to treat the epidemic in the hardest hit countries, we must prevent major epidemics in the most populous countries."

Today, Piot noted, the prevalence of HIV infection among adults (people aged 15 to 49) in China, India and Russia is low compared with sub-Saharan Africa.

In all three countries, the HIV epidemic is so far confined almost entirely to high-risk groups -- injecting drug users, and prostitutes and their customers.

When it breaks out of those groups into the general population of heterosexual adults is when the numbers of infected people begins to rise exponentially.