An escalation in violence in Somalia has left many too frightened to seek treatment for cholera, medical charity Medecins Sans Frontieres has warned.
Not all patients can access a hospital or clinic
It says there could be a "humanitarian crisis" as people flee fighting between Ethiopian-backed troops and Islamist insurgents in the capital Mogadishu.
MSF has called on those fighting to respect the work of aid agencies.
Just over 1,200 patients have reached the cholera treatment centre - but MSF say this is the "tip of the iceberg".
Cholera is an intestinal infection caused by bacteria - and is often linked to contaminated supplies of drinking water.
It causes severe diarrhoea and vomiting, and patients - particularly children and the elderly - are vulnerable to dangerous dehydration as a result.
Cholera proves fatal in about 1.5% of cases.
James Bakti, MSF's assistant medical co-ordinator, warned that the fighting is making things difficult for those seeking and giving treatments.
"We still have cholera patients coming to the treatment centre, but the numbers are reducing because of the insecurity.
"And we have had deaths because people are coming late. The mortality is increasing."
Henry Rodriguez, medical co-ordinator for MSF in Somalia, said: "The health facilities which normally work in Mogadishu are not able to do so due to the violence.
"It's also impossible for us to carry out preventative measures like supplying clean water or establishing oral rehydration points which would help in preventing the spread of the epidemic."
In addition, hundreds of thousands are fleeing to safer areas of the country.
But MSF officials warn they are heading to places with a general lack of sanitation, where there are no latrines and very little access to drinkable water.
They are also worried that basic health services and shelter cannot be provided.
Most people fled without food or with very few supplies and MSF say rises in local prices might create further health problems as nutrition deteriorates.
In Afgooye, a former university campus west of Mogadishu, about 50,000 people are seeking refuge.
Others have found shelter in the Lafolle university buildings, some 10 kilometres from Afgooye, while others are living under precarious shelters or even under trees alongside roads.
Among these people there are already signs that the health situation is rapidly deteriorating - 401 cases of watery diarrhoea have been reported, including 13 fatalities.
Charity staff managed to reach the area on the April 17 and supplied medicines to existing health facilities.
They are also working with other organisations to provide safe drinking water.
But MSF says despite bringing in 48,000 litres of clean water every day to those in Afgooye, it is still too little.
In recent weeks, the area has seen an influx of between 800 and 1,000 people a day, and more are expected to come if the violence continues in and around the Somali capital.
Hugues Robert, head of the emergency desk, at MSF, said: "If the present situation in Afgooye was to last, we could be faced with a critical humanitarian crisis very rapidly," he said.