But now Ratana has a second job, also born of dire necessity. She is one of seven Aids educators who are teaching other prostitutes in the city's red light districts how to practise safe sex.
Although war and isolation from the rest of the world blocked the spread of the disease into Cambodia when it first became prevalent in Asia, the country now has the world's fastest-growing Aids problem.
The first case was only diagnosed in 1991, but now four out of 10 prostitutes in Phnom Penh are HIV positive. This figure was gleaned, controversially, by anonymous blood-testing of prostitutes who came to a health centre and few actually know they are carrying the virus.
Nobody is quite sure how many prostitutes there are working in Phnom Penh, but the number certainly runs into thousands. Popular myth has it that most of the clients are foreign, but most health workers believe this is untrue.
Ratana's job, for which she is paid a princely Pounds 20 per month on top of her normal earnings, is to visit brothels and to show the workers a series of flip-chart pictures about how Aids is transmitted. She is often accompanied by staff from the Toul Kork Dike Clinic, which was set up in 1993 with the help of a British doctor who was working for Voluntary Service Overseas.
Unlike most Cambodians, Ratana is plump, and gold teeth flash in her mouth as she talks. Like so many others here she has known starvation, and she carries her weight with pride. But when she begins to recount how she came to be here, her eyes well with tears.
"My story is a very sad one," she says.
Born into a family of rice farmers in Pursat, north of the capital, she saw her mother die of hepatitis during the famine which came at the end of the four-year Khmer Rouge regime in the 1970s. There was no medicine and, with her stomach swollen from malnutrition, she could not survive the illness.
Married in 1979, aged 23, Ratana had seven children in as many years before her husband left her for someone else. She struggled on for a few years growing and selling rice before another disaster struck: travelling to Phnom Penh with 1,000 kilograms of rice - a whole year's harvest - she was stopped by some of the country's notoriously corrupt policemen and her goods were stolen.
A friend who was with her promised to help but instead Ratana found herself enslaved to a brothel - the owner had probably paid around Pounds 200 for her services.
Eventually she paid off the debt and went home but she found her family near to starvation again and was forced to return. Now she lives in a brothel and a third of her Pounds 4.80 daily earnings go in rent. Most of the rest is sent to her sister, who looks after her children and who thinks Ratana works in a hotel.
She wants to stop but her family needs the money, she says, and she could never find another job that paid as much.
So Ratana improves her lot by working to combat Aids.
"I think about HIV but I need the money because I don't want my family to be starving. I like to do this at the clinic, it makes me very happy," she says.
One of the workers who accompanies Ratana on her visits to the local brothels is Julie Forder, a VSO nurse and health visitor who works as an outreach co-ordinator with Cambodia's national Aids committee.
Women here are not used to talking about sex, she says, but attendance at the clinic is increasing. The majority of the women who attend know about HIV, though some have not heard of it.
"Culture is an abiding problem because it doesn't allow women to talk about these things. But if they don't learn about it how can they make themselves safe?"