The young and hip at ground zero of the Aids epidemic meet in the Botswanan city of Francistown, drink and pair off under the knowing gaze of bartender Brian Khumalo. Sometimes they first buy a three-pack of condoms from the box he keeps by the liquor, sometimes not.
Night after night they return for the carefree, beery vibe of his bar, with the same partners or new ones, creating a web of sexual interaction. A growing number of studies single out such behaviour - in which men and women maintain two or more simultaneous relationships - as the most powerful force propelling a killer disease through a vulnerable continent.
This new understanding of how the Aids virus attacks individuals and their societies helps explain why the disease has devastated southern Africa while sparing other places. It also suggests how the region's Aids programmes, which have struggled to prevent new infections even as treatment for the disease has become more available, might save far more lives: by discouraging sexual networks.
"The problem of multiple partners who do not practise safe sex is obviously the biggest driver of HIV in the world," said Ndwapi Ndwapi, a top government Aids official in Botswana. "What I need to know from the scientific community is, what do you do? . . . How do you change that for a society that happens to have higher rates of multiple sexual partners?"
Mr Khumalo, 25, tall, with a crooked-toothed smile, described the problem as he pointed to a woman in a corner booth. "She's new around here, so every guy is going to talk to her," he said. "She will be with me today. Tomorrow she will be with my best friend. And I will be with somebody else."
The barman moved from the capital, Gaborone, to Francistown in March last year, finding a city of 85,000. On the first night he slept with a woman he had just met. He did the same on the second night, the third, the fourth. Although he used condoms each time, he said, an alarmed friend soon drove him to the white, low-slung buildings of Francistown's biggest Aids clinic. "I saw thousands of beautiful women going to get pills," Mr Khumalo recalled.
It scared him, but not enough. By the end of the year he had slept with more than 100 women, he said.
The number of sexual partners is not the only factor that increases the risk of Aids. The most potentially dangerous relationships, researchers say, involve men and women who maintain more than one regular partner for months or years. In these relationships, more intimate, trusting and long-lasting than casual sex, most couples eventually stop using condoms, studies show, allowing easy infiltration by HIV.
Researchers increasingly agree that curbing such behaviour is key to slowing the spread of Aids in Africa. In a report published in July, southern African Aids experts and officials listed "reducing multiple and concurrent partnerships" as their first priority for preventing the spread of HIV in a region where nearly 15 million people are estimated to carry the virus - 38% of the world's total.
A second key factor helping the virus spread through southern Africa is low rates of circumcision. Before European colonialists arrived, most tribes removed the foreskins of teenage boys during manhood rituals. Those rites were discouraged by missionaries and other westerners who regarded them as primitive.
Dozens of studies, including three experimental trials conducted in Africa in recent years, show that circumcised men are much less likely to contract HIV because the most easily infected cells have been removed.
West Africa has been partly protected by its high rates of circumcision, but in southern and eastern Africa - which have both low rates of circumcision and high rates of multiple sex partners - the Aids epidemic became the most deadly in the world.
"That's the lethal cocktail," said a Harvard University epidemiologist, Daniel Halperin, a former Aids prevention adviser in Africa for the US government. "There's no place in the world where you have very high HIV and you don't have those two factors."
