Aid groups are trying to halt the spread of cholera in Haiti's capital Port-au-Prince.
Makeshift camps full with earthquake survivors are ripe ground for the epidemic to take hold.
The outbreak began in the Artibonite River valley in central Haiti in mid-October and initially seemed to have been contained.
However, the death toll from the chronic disease has since soared to 724.
Some 115 cases and a first death have been confirmed in Port-au-Prince, while reports came in from northern Haiti of villagers on foot dying on the way to hospital and taxi drivers too scared to help.
Most of the estimated 1.3m Haitians living in refugee camps are in tent cities around the capital and water-borne cholera could spread easily in the unsanitary conditions where supplies are shared for cooking and washing.
'The epidemic of cholera, a highly contagious disease, is no longer a simple emergency, it's now a matter of national security,' said Gabriel Thimote, the director of the Haiti's health ministry.
Haitian authorities have been warned to prepare for the worst if cholera spreads in the capital, which is still largely in ruins after a January earthquake that killed more than 250,000 people.
'Port-au-Prince is a large urban slum with very poor water and sanitation conditions. This is ripe for the rapid spread of cholera. We have to be prepared for it,' said UN health official Jon Andrus.
Leading humanitarian organisation Doctors Without Borders (MSF) said it was extremely concerned at the increase in the number of patients with cholera-like symptoms in the capital.
'The increasing numbers of cases of suspected cholera in our facilities throughout Port-au-Prince are certainly alarming,' said Stefano Zannini, MSF head of mission in Haiti.
Dublin woman Dr Louise Ivers, who works in Haiti with the organisation, 'Partners in Health', also said the outbreak is likely to get worse.
