Dozens of hungry children in famine stricken Somalia are dying from the fast spreading disease.
The UN said on Friday that cases of the disease, linked to dirty water, had reached epidemic proportions.
At least 181 people have died from cholera this year in just one hospital in Mogadishu, and half the victims were younger than two.
"The concern here is the root cases of cholera, and that is related to water and sanitation,” said the United Nations’ Michel Yao.
And with more people on the move as they journey far from home in search of food, aid agencies fear it could quickly spread as thousands flee famine in the south.
"With cholera, everyone is at risk, the World Health Organisation’s Tarik Jasarevic added. “But the conditions are perfect for transmission now in Mogadishu," he told Agence France Presse.
Children are particularly vulnerable, the UN's agency for children said.
"The massive influx of people into Mogadishu and the rising number of cases of acute watery diarrhoea in crowded urban areas places malnourished children at grave threat from cholera, which is a deadly and contagious disease," said the agency’s Marixie Mercado.
"Right now you have around half a million children at imminent risk of death if they do not get (food) help within weeks.
"But beyond being a malnutrition crisis, it is also a crisis for child survival generally because children who are acutely malnourished are that much more susceptible to cholera... all in the context of massive displacement and poor water and sanitation," Mercado said.
At least 100,000 Somalis have fled to Mogadishu from the countryside in the last two months. Even more have travelled to sprawling drought-affected refugee camps in Kenya and Ethiopia. About 1,400 Somalis arrive in Kenya's Dadaab settlement every day, pushing the number of recorded refugees past 400,000. Another 38,000 people are still waiting to register.
Now with the three Dadaab camps massively overcrowded most people arriving now have to settle on the edges, far from latrines and water points. Unless these families are moved to better areas before the next rains, fresh cholera outbreaks are likely, aid agencies say.
Meanwhile in southeastern Ethiopia, the Dollo Ado refugee camps which host 120,000 Somali refugees, have had to bring in emergency measles vaccinations. The UN said 93 cases had been reported in the camps, with three deaths.