There is a rustle behind the bed curtains and Regina Ombita comes out in a bright red T-shirt with the slogan: I Am HIV Positive. Actually, the T-shirt is a lie. Although Regina's husband is suspected to have died from HIV/Aids he was never tested. She herself tested negative.

She wears the T-shirt as a challenge, a come-on almost. "I am happy to wear it, because then some people come and ask me questions. Some say, if Regina can wear these clothes, there is no fear."

Once dressed Regina, 40, strides out of her hut to visit her "clients". She is a volunteer for the medical charity Médecins sans Frontières in western Kenya, and her clients are HIV-positive neighbours. Her job is to give advice, to see whether clients are taking their medicines, to act as eyes and ears for the medical staff.

Ducking through the low doorway of Charles Wanneyo's hut, Regina finds him lying on a mattress on the floor, wrapped in a grey blanket. He has an upset stomach, and Regina asks whether he has been drinking boiled water. Charles's 18-year-old son, his main carer, was supposed to boil the water for him but has been sent out of the village on an errand. His brother, who is sitting alongside, promises to fill the breach.

Charles is taking drugs for tuberculosis, which are almost finished. "Tomorrow you should make sure you go to the clinic," Regina tells him. "Go early, because the clinic will be busy."

While HIV/Aids is a dramatic problem for Africa, the discomforts of living with the virus are much like the problems of any chronic illness: how to find a carer, how to get to the clinic, how to cope with taking large quantities of drugs. In public clinics, overworked doctors and nurses have little time to answer these questions. Volunteers can fill that gap.

The advice the volunteers give can be as simple as good nutrition. Some villagers believe that eating well means saving until they can afford processed food such as sliced bread. Volunteers encourage them to eat eggs laid by their own chickens, and garden vegetables.

Busia lies near Lake Victoria, where the abundance of fish is both a blessing and a curse. "Money sells HIV very fast," Regina said. "A farmer plants a seed and waits for it to grow, but a fisherman catches a fish and sells it the same day." Fishermen often trade part of their catch for sex, often unprotected. The prevalence of HIV/Aids among adults in the Busia district is estimated at 16%.

The practice of "widow inheritance", in which a woman whose husband dies marries one of her husband's relatives, gives the Aids virus a way to travel rapidly through a community. When Regina's husband died, she was approached by a relative who wanted to "inherit" her. "I told this man, the first thing we shall do before taking each other is to go for the test. He did and the man was positive. That saved me a lot."

Not everyone is so wise. Helen Usiku, a basket weaver, is married to a fisherman, Justus. Both look healthy, but Helen has lost several children in infancy and is pregnant again. Both are HIV positive. The virus is believed to have entered their household when Justus "inherited" an HIV-positive widow. Helen was embarrassed about having conceived. "I felt afraid," she told Regina. "In the hospital they said that if you are HIV-positive you are not supposed to conceive and deliver a baby, you are supposed to have protected sex." Regina urged the couple to have their baby in a clinic, where medical staff can reduce the chances of HIV transmission from mother to child.

Regina became a volunteer when her husband was dying, and MSF volunteers gave her counselling. There are now 144 volunteers in Busia district. By monitoring patients and helping their carers, they have improved the quality of life for more than 2,000 HIV-positive people and vastly reduced the pressure on local hospitals. "Before we had the volunteers, our hospitals were full," said Beatrice Misoga, an MSF nurse. "Now the hospitals have been relieved."