AIDS is now the leading cause of death in Africa. In Harare, Zimbabwe's capital, four out of every 10 adults are HIV-positive. The effects of that are difficult to imagine -- impossible to imagine, in fact, since the full force of the epidemic has yet to hit. But it becomes clearer all the time that AIDS will have vast and long-term consequences for many societies.

In the United States, fewer people are dying of AIDS, thanks to new drug regimens. But even here the virus isn't defeated. Last year in North America, there were some 44,000 newly reported infections.

These are some of the gloomy tidings from Peter Piot, a Belgian physician and microbiologist who directs the United Nations program on AIDS. Piot actually came to town with some good news of sorts -- more on that below. But if you want gloomy, he can do gloomy.

Last year, according to the World Health Organization, about 54 million people died. AIDS was responsible for 2.3 million of those deaths -- more than malaria, or tuberculosis or lung cancer. AIDS is now among the top five killers in the world, and in many places the epidemic is still advancing.

Its toll is especially high in developing countries, most of all in Africa, where the virus originated. In Botswana, for example, children born in the next few years can expect to live, on average, to just past their 40th birthday; were it not for AIDS, their life expectancy would be 70.

The picture of reduced life spans and orphaned children is repeated throughout much of southern and eastern Africa. Nearly a million North Americans, 1.4 million Latin Americans and 7 million Asians also are living with HIV, and India and China may still lie ahead in the epidemic's path.

AIDS is unusual, not only for how long the epidemic takes to crest but also for claiming victims at their most productive age, both as workers and as parents. But millions of children are threatened, too.

No sane person could speak of optimism in the face of such numbers, but Piot sees two grounds for encouragement. He goes so far as to claim "a real turning point" in the past six months or year.

For the first time, he says, many countries and many heads of state that refused to acknowledge the epidemic are now confronting AIDS openly. A model is South Africa, where AIDS infections continue to grow at a frightening pace.

"Although AIDS has been a part of our lives for 15 years or more, we have kept silent about its true presence in our midst," then-president Nelson Mandela acknowledged just last December. He urged his compatriots to "break the silence by speaking openly and publicly about AIDS, and by bringing an end to discrimination against those living with AIDS."

In addition, Piot says, "we now have good proof of what works and what doesn't work, and on a large scale." Here he is referring to education and prevention in countries such as Uganda and Senegal, an African country that has kept HIV incidence below 2 percent.

One key, he says, is commitment by top leaders; both countries, in addition to receiving foreign aid for their programs, have spent from their own budgets. Another is early education that teaches not only safe sex but also emphasizes self-respect and respect for partners.

Few of these countries are stalemated by the familiar American debate between abstinence and sex education, Piot says. As it turns out, programs that emphasize values, even without preaching abstinence as the exclusive answer, seem to promote abstinence. In Uganda, for example, the average age of first sexual experience has been delayed by two years.

It's disturbing that part of what works in the United States and Western Europe -- drug treatments for those already suffering from AIDS -- isn't available for most of the developing world. Most people there can't afford the drugs, and many of the hardest-hit countries don't have the clinics and laboratories to help patients manage the complex schedules and combinations of prescriptions.

Piot isn't satisfied with this situation. There are developing countries, such as South Africa, where the health establishment could manage the drug regimens if prices were lower, he says.

But when the U.N. AIDS chief appeals for more funding -- a quest that, not surprisingly, occupies much of his time -- it is not primarily for drugs. "The best investment is in prevention," he says.

Funding to combat AIDS hasn't nearly kept pace with the epidemic's progress, Piot's numbers show. The United States is a leading donor in dollar terms but, as with most foreign aid, a laggard when assistance is measured as a share of the national economy.

To develop a vaccine will take not only more funds but more focused leadership, Piot says. "Not enough is being done, that's for sure," he says. At the current pace, he estimates that a vaccine is more than a decade away, "unless there's some miracle."

And without greater commitment to prevention, what course will the epidemic take? Piot offers no forecasts. All the computer models, he says, predicted that the epidemic would peak in 1991. Now, he says, the models aren't sure what to say.

The writer is a member of the editorial page staff.