UN medical experts in the western Sudanese region of Darfur have expressed concern over an outbreak of Hepatitis E which has killed 22 people.
The UN fears a lack of security will prevent refugees returning home
A doctor working for the UN Population Fund, Henia Dakkak, told the BBC the disease was spreading quickly because of poor sanitation in the camps.
Ms Dakkak said pregnant women were most at risk of infection.
About a million black Africans have been driven from their homes in Darfur, mostly by pro-government Arab militias.
The Sudanese government denies any involvement in atrocities, saying it is trying to improve security in the region.
But on Wednesday UN special envoy to Sudan Jan Pronk urged the government to do more to end the 18-month conflict.
"I don't see a voluntary return of more than one million
displaced people to their villages to start in the next
three weeks because of lack of security," he said.
The UN Security Council last month approved a resolution urging Sudan to put a stop to the violence by the end of August.
Disease 'will spread'
Ms Dakkak said the Hepatitis E outbreak was being exacerbated by the fact that sanitary conditions in the refugee camps were sub-standard, with people receiving less than two litres of water per day each.
She described the situation as "alarming", and said the disease was likely to spread very quickly.
Up to 50,000 killed
More at risk from disease and starvation
Arab militias accused of ethnic cleansing
Sudan blames rebels for starting conflict
More money was needed from international donors to get supplies to the camps and carry out water chlorination, she said, adding that the situation was no better across the border in Chad, where many of the refugees have fled.
A senior UNHCR official, Jean-Marie Fakhouri, is due to visit camps in eastern Chad on Thursday.
Many refugees there are without adequate shelter and roads have been closed as a result of recent heavy rains.
Humanitarian relief supplies are unable to reach the biggest camp, Breidjing.
The government says it has sent thousands of extra troops to Darfur in an effort to rein in militiamen and protect civilians.
But the report by the US-based Human Rights Watch says the militias are still attacking civilians.
The document give details of incidents said to have taken place in July.
It also accuses the government of incorporating militia fighters into the police and other state security forces.
Last week, Sudan and the UN agreed on a plan to tackle the crisis, which included setting up safe areas around certain towns and villages.
A UNHCR spokesman in Geneva said many people had been shot by Janjaweed raiders after returning to their villages.