THE VACCINE.(search for AIDS vaccine in Africa)

At forty-one, Hala has five children and eight grandchildren. Her first husband left when their second child was born. Her second husband died of AIDS nearly twenty years ago, in the earliest days of the epidemic. Hala often tells people that she sells charcoal, doughnuts, or cooking oil on the streets, but that isn't true. She is a prostitute, who has spent nearly half her life working out of a wattle hut in Pumwani, one of Nairobi's most crowded--and violent--slums. On an average day, she might see ten men, most of them truck drivers from Tanzania. Her "office" has just enough room for a single bed, a stool, a customer, herself, and a wicker basket filled with condoms. The basket is a recent addition; only in the past year or so have her clients agreed to use condoms with any regularity.

None of these details make Hala unusual. Despite the severity of the AIDS epidemic, Kenyans have only just begun to speak openly about the disease, and the epidemic has certainly done little to deter prostitution. As many as two and a half million people in Kenya, one in six adults, are infected with H.I.V. In Pumwani, more than ninety per cent of prostitutes--and many of their clients--test positive for the virus. Hala has engaged in unprotected sex with hundreds of H.I.V-positive men. Her best customer--a man who visited her regularly for seventeen years, and never used a condom--recently died of AIDS. Remarkably, though, she has never become infected.

The day I was introduced to Hala, at a clinic not far from where she lives, she was draped in black robes and wore a purple shawl with gold piping down the sides. She is a handsome, businesslike woman, and she is completely baffled by her fate. "I have no idea why I of all people have been spared,'' she told me. "But if my luck can be useful to the doctors then I will be grateful."

Hala belongs to an increasingly famous cohort of research subjects, known by AIDS experts throughout the world as "the Nairobi prostitutes.'' In the late nineteen-eighties, the Canadian infectious-disease expert Francis Plummer noticed something startling: in a study of two thousand Nairobi prostitutes, as many as two hundred remained uninfected, despite years of constant high-risk behavior. Later, when Plummer and his colleagues examined the data more closely, they realized that if the prostitutes didn't become infected within five years of their first exposure to the virus, they were unlikely to become infected at all.

How, exactly, were these women protected when millions who engaged in the same behavior fell ill and died? It couldn't have been luck; nobody gets lucky a thousand times in a row. Nor was it good nutrition; the women often lived on plantains and rice, and many were weak, undernourished, and sickly. Plummer concluded that these women harbored a rare defensive weapon within their immune systems. To many vaccine researchers, the implications were thrilling: if they could identify that weapon and somehow bottle it, they might help to end the world's most devastating epidemic. "We all held our breath for a while,'' Job J. Bwayo, the director of the International AIDS Vaccine Initiative in Kenya, told me when I went to see him at the University of Nairobi. "Nobody expected a simple solution to come from it, but we have all kept hoping that somehow the girls will provide the key."

The search for a solution has become desperate. Twenty-one years after the first cases of "slim disease," as AIDS was initially called in Africa, appeared in a Ugandan village on the shores of Lake Victoria, scientists are only marginally closer to producing a successful vaccine than they were when they identified the virus that causes it. A great deal has changed for people with AIDS in those two decades: medicines are now routinely available throughout the developed world. But, of the forty million people who are living with H.I.V., less than five per cent have access to them. Because that percentage will not change dramatically in the next decade, the world has never needed a medical intervention more urgently than it needs an AIDS vaccine today.

Thanks in part to a United Nations special session devoted to AIDS, the plight of Africa has finally gained at least a measure of attention. Last fall, I watched at a clinic in Nairobi as a farmworker received a new kind of H.I.V. test, one that provides almost instant results. A nurse took a few drops of his blood, the man sat quietly on a bench for half an hour, and then he had his answer. Even a year ago, this man, and tens of thousands like him, would have had to make a second journey to the clinic--often on foot--to get the test results; many would not have been able, or willing, to do that. While I was in Nairobi, I attended a support group for infected mothers, visited well-maintained clinics in the most fetid slums, and was surprised to see billboards on the main highway urging precautions. After years of denial, even politicians acknowledge that AIDS poses a serious threat to the future of the nation. In Uganda, where President Yoweri Museveni long ago established himself as the African leader most willing to speak openly about the disease, the epidemic has ebbed. In each of the past ten years, there have been fewer new H.I.V. infections than in the year before, a feat managed by no other African country. Although six per cent of the adults in Uganda remain infected--a toll that would be horrifying almost anywhere else on earth--the figure a decade ago was more than twenty per cent.