Sex is no longer the main driver of Aids in Africa. It is love, and trust.

This autumn I spent five weeks travelling through the most captivating continent on Earth, examining the epidemic that has laid waste to it. As I journeyed through bars and brothels, schools and hospitals, clinics and research centres, one phrase echoed in my head. I talked to scores of doctors, sex workers, tribal leaders, patients, civil servants, mothers, social workers and campaigners, assisted by a no-strings grant from the European Journalism Centre.

But it was the words of Jane Thiomi, manager with a Kenyan HIV organisation called LVCT in Nairobi, which kept coming back to me.

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She was explaining how, among sex workers, it was not their clients who infected them, but their boyfriends. “It is love that makes them vulnerable,” she said.

In its fourth decade, the epidemic, which has claimed over 20 million lives so far, has changed. Unlike other diseases which wreak devastation across the continent such as malaria and tuberculosis, Aids is wholly avoidable. People know better than they did what they need to do to protect themselves. They know the importance of abstinence, faithfulness and condoms.

What they have not learnt to deal with is love. Long-term relationships between men and women involve commitment and trust and it is fear of undermining that trust that prevents many from seeking the protection – whether condoms, male circumcision, or treatment with antiretroviral drugs – that they need.

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While Western leaders proclaim the end of Aids, bolstered by research showing antiretroviral drugs reduce HIV transmission by 96 per cent, few in Africa share their optimism. Huge advances have been achieved in rolling out the drugs – taken by 10 million people –and new infections have fallen by a third since 2001.

But no one knows how much of that is due to behaviour change, drug treatment or to the natural course of the epidemic – newly emerging viruses always cull the most susceptible first. The fall has slowed in recent years and in Uganda new infections have begun to rise again. The drivers of the epidemic in Africa are strengthening, not weakening. As economies grow and men become better-off they can afford to keep mistresses or use sex workers. Continuing gender inequality keeps women dependent on men, forcing many who lack a male breadwinner into sex work.

The major donors are losing faith in efforts to change sexual behaviour. In Uganda, the US Centres for Disease Control organisation has closed its behaviour change research centre in Entebbe. In Kisumu, Kenya, a well-regarded youth project called Tungaane shut its doors after donors switched to biomedical interventions such as male circumcision.

Biomedical interventions require behaviour change too. As with condoms, men must be persuaded to volunteer for circumcision and HIV-positive pregnant women must be persuaded to accept antenatal testing and drug treatment. In each case what prevents them doing so is love.

Condoms, the single most effective preventive measure against HIV infection, were once uncommon in Africa. Today they are widely used in casual affairs and with sex workers. But their use in long-term relationships is close to zero – it signals mistrust.

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“If a man uses a condom, the woman will ask him, ‘Are you sick?’ If a woman uses one, he will ask her ‘Are you having other men?’ Most people have negative attitudes to condoms. There is very little use by couples,” said Molly Businge, chief nurse at the Kawaala Health Centre in Kampala.

With male circumcision, the message about its protective effect has got through. Parents bring their children and teenage boys to queue up for the surgery. But the over 25-year-olds stay away. They have wives and girlfriends – how do they explain why they need circumcision? Fear of undermining their relationships holds them back.

A pregnant woman who discovers she is HIV positive must start drug treatment immediately, and continue for life. She must bring her child back for testing after birth and may have to give it drugs, too. And she must break the news to her husband who may reject her. Many fear this so much they throw the drugs away.

Among the lessons I learnt on my journey are that there is not one HIV epidemic in Africa but hundreds, each with its own drivers. That nothing can be achieved without involving specially trained “peer educators” – members of the community being targeted to help spread the message – a strategy that is now being adopted across the continent. And while starting people on drugs is a huge achievement, keeping them on them is an even tougher challenge.

But one area of research has been neglected – trust. If we understood it better we might understand how to help couples protect themselves more effectively from the holocaust that has swept Africa. In Kenya, almost half (44 per cent) of the 100,000 new infections a year occur in stable relationships while just a third are linked to high-risk groups such as sex workers.

Nduku Kilonzo, head of the Kenyan HIV organisation LVCT, said: “We are not investigating the nature of trust. What leads a couple to abandon condoms for example? It’s a key thing we avoid. We need to start thinking about it.”