A week after the flooded Mozambican port of Beira was hit by Cyclone Idai, cases of cholera have been recorded, a humanitarian aid group said on Friday.
The International Federation of Red Cross and Red Crescent Societies (IFRC) warned of the risk of other outbreaks, already noting an increase in malaria.
The storm has so far killed 557 people across Mozambique, Zimbabwe and Malawi, but the death toll is expected to rise.
Idai made landfall near Beira with 177km/h (106 mph) winds on 14 March.
Aid workers are slowly delivering relief but conditions are said to be extremely difficult, with some areas completely inaccessible and a scarcity of helicopters.
Some 1.7 million people are said to be affected across southern Africa, with no electricity or running water in areas where homes have been swept away and roads destroyed by the floods.
Cholera, which is endemic in Mozambique, is spread by water contaminated by sewage, and can kill within hours if left untreated.
Many people are said to have not yet received emergency rations, with some still clinging to rooftops and trees.