Farming in Africa has declined at an alarming rate since the start of the Aids epidemic, scientists say.
Some 40% of people in Swaziland are HIV positive
The virus is ravaging agriculture, with the areas of cultivated land dropping in parts of Kenya by 68%.
The land used is being planted with less nutritious and less profitable crops, they said at the British Association of Science Festival.
The report comes as aid agencies warn that some 10m people need food aid in southern Africa.
Some 80% of Africans derive their livelihood from farming, so it is vital to the continent's economic growth.
Southern Africa has some of the world's highest rates of HIV infection, with between 30% and 40% of the adult population HIV positive.
Those sick with Aids are unable to work in the fields and are consequently first affected by even a slight lack of rain, says Oxfam's Malawi programme manager Nellie Nyangwa.
The new report by the System-Wide Initiative for HIV/Aids and Agriculture, based at the Africa Rice Centre, found that people were switching from cash crops such as coffee to less labour intensive ones such as maize or cassava.
In Rwanda, there has been a 60-80% fall in farm labour.
In Burkina Faso, 20% of rural families reduced their agricultural work or even abandoned their farms because of Aids.
The aid agencies are urging donors not to ignore the lessons of Niger's food crisis, when early warnings of hunger went unheeded and little was done until television cameras showed children starving to death.
Oxfam says people could face food shortages in a belt of countries including Malawi, Zambia, Zimbabwe and Mozambique.
Zimbabwe worst hit
Ms Nyangwa told the BBC's Network Africa programme that many Malawians were not eating the recommended three daily meals, while children were dropping out of school to help their parents gather food.
"At the moment, the situation is not as serious as Niger but if we let it go for another three, four or five months, it is going to deteriorate and will be like the Niger situation," she said.
NEEDING FOOD AID
She said Malawi's government was acknowledging the severity of the situation and had set up a food crisis fund, to which local companies and individuals were contributing.
The UN's World Food Programme says the shortages would increase when the traditional lean season starts - when this year's harvests run out - from December.
It says it has a funding shortfall of $191m.
Southern Africa was weakened by severe food shortages leading to major international appeals in 2002, and drought this year has been worse than it was then, aid agencies say.
Oxfam began to distribute food in Malawi this week, but they say they need far more.
Zimbabwe is the worst-hit country, with up to 4m people going hungry.
Donors and government critics say the seizure of white-owned land the slum clearances have increased hunger levels.
This is denied by the government, which accuses western countries of withholding aid to punish President Robert Mugabe for his land reform programme.