The health ministry in Port-au-Prince has confirmed that the country's cholera outbreak has reached the Haitian capital.
Doctors are treating 73 people for the disease, amid fears that it could spread across the quake-hit city.
Meanwhile, the Pan American Health Organisation (Paho) says it expects tens of thousands more Haitians to catch cholera in the next few years.
The health ministry says so far 583 people have died in the epidemic.
Dozens of suspected cases are also being investigated in Port-au-Prince, which has feared an outbreak since October.
Across the country, more than 9,000 people are being treated for symptoms of the disease, the health ministry said on Tuesday.
Health minister Ariel Henry said that a sizeable outbreak in Port-au-Prince now appeared likely.
"It's coming," Mr Henry told the AFP news agency.
'Foothold'
The deputy director of the Pan American Health Organisation (Paho), Jon Kim Andrus, said his organisation expected cholera transmission in Port-au-Prince "to be extensive".
Mr Andrus said Paho and other organisations were preparing for a long-running epidemic likely to infect tens of thousands of people across Haiti over the next few years.
"The disease now has a foothold in Haiti's river system," he added.
Mr Andrus said that while it is impossible to predict the development of the epidemic in Haiti, for planning purposes Paho has extrapolated from the 1991 cholera outbreak in Peru, which spread to 16 Latin American countries over six years.
Comparing the population figures of Peru and Haiti and factoring in the number of cholera cases in Peru during the outbreak in the 1990s, the organisation thinks that 270,000 people in Haiti could fall ill.
"But the number in Haiti could be even higher, because the conditions there are worse than in Peru," Mr Andrus told the BBC.
So far the Haitian death rate has also been higher than that registered during the Latin American epidemic, he added.
While trying to educate the population about prevention and treatment of the disease, reducing the fatality rate of the disease in the Caribbean country remained a top priority, Mr Andrus said.
'High alert'
Paho said it was also on "high alert" over the risk of cholera spreading to the neighbouring Dominican Republic and has sent experts to prepare for a possible outbreak in that country.
Haiti's health minister said a major cholera outbreak in the capital was likely
Mr Andrus warned there could be an "upsurge" in cholera cases in Haiti in the coming days as a result of water and sanitation problems caused by Hurricane Tomas at the end of last week.
"We have every reason to expect that the widespread flooding has increased the risk of cholera spreading."
The water-borne disease has already spread to half of Haiti's 10 regions.
The storm at the end of last week left 20 people dead, with 36 injured and 11 missing, officials said.
Aid agencies say the main concern is that the flooding could result in people lacking access to basic sanitation and being forced to drink contaminated water.
The hurricane passed without destroying the tented camps in and around Port-au-Prince, which house about 1.3 million survivors of January's earthquake.
Aid workers say those living in the tent cities have better access to toilets and clean drinking water than the residents of some of the capital's long-standing slums, says the BBC's Laura Trevelyan in Port-au-Prince.
But if more cases are confirmed, the outbreak could threaten an estimated 2.5 to three million people in Port-au-Prince.
Cholera itself causes diarrhoea and vomiting, leading to severe dehydration. It can kill quickly but is treated easily through rehydration and antibiotics.
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CROIX-DES-BOUQUETS, Haiti -- You name it, Camp Corail has got it. And Camp Obama does not.
The organized relocation camp at Corail-Cesselesse has thousands of spacious, hurricane-resistant tents on groomed, graded mountain soil. The settlement three miles (four kilometers) down the road -- named after the U.S. president in hopes of getting attention from foreigners -- has leaky plastic tarps and wooden sticks pitched on a muddy slope.
Corail has a stocked U.N.World Food Program warehouse for its 3,000-and-counting residents; the more than 8,500 at Camp Obama are desperate for food and water. Corail's entrance is guarded by U.N. peacekeepers and Haitian police. Camp Obama's residents put up a Haitian flag to mark their empty security tent.
The camps, neighbors in the foothills of a treeless mountain, are a diptych of the uneven response to Haiti's Jan. 12 earthquake. More than $12.7 billion has been pledged by foreign governments, agencies and organizations, including $2.8 billion for humanitarian response and another $9.9 billion promised at the March 31 U.N. donors conference.
In one camp, which dignitaries and military commanders visit by helicopter, those billions are on display. A short hop down the road, they barely register.
''We've heard the foreigners have given a lot of aid money. But we're still living the same way as before, and we're still dying the same way as before,'' said Duverny Nelmeus, a 52-year-old welder-turned Camp Obama resident-coordinator.
Haiti's needs are still enormous, but more than 100 days after the quake, the plan for dealing with them is unclear. Even the death toll is confusing: Government estimates hovered around 230,000 until the U.N. donors conference when, without explanation, the total jumped to 300,000.
There are officially 1.3 million people displaced by the magnitude-7 earthquake. Hundreds of thousands have massed in settlement camps that, like Camp Obama, sprouted with little or no planning. These Haitians live in makeshift tarp homes and shanties, govern their affairs with self-formed security committees and make do with whatever aid arrives.
It was said early on that nearly all the displaced needed to be moved ahead of the arriving rainy season to carefully planned camps like Corail. But it took months to procure land.
By March, aid officials decided instead that people should start going home, saying thousands of houses are still habitable or can be repaired.
It was even better, they said, for most to stay where they were: Agencies deemed just 37,000 people in nine camps at high risk for flash floods, said Shaun Scales of the International Organization for Migration.
But many people are not moving, nor do they want to stay where they are.
Persistent aftershocks and rumors of more to come -- President Rene Preval warned of an impending earthquake at a news conference this month -- are keeping people from going back. Private landowners and schools are threatening to evict squatters. Those who remain are suffering.
What they want is a better option. And for a few lucky people, right now, that's Corail. The product of a coordinated effort by aid agencies, the United Nations, the U.S. military, the Haitian government and other entities, it has sprung up seemingly overnight on a cactus patch where the Cite Soleil slum meets the suburb of Croix-des-Bouquets.
There was little here but a few concrete homes, disorganized camps and brush until a few weeks ago, when Preval announced that the government would seize -- with compensation for the owners -- 18,500 acres (7,490 hectares) of the arid land.
Authorities began moving people in immediately, even before services were in place. Croix-des-Bouquets officials say they were unprepared for the onslaught. Aid groups Oxfam, World Vision and CARE criticized the rush as violating human dignity.
Now ecstatic arrivals are streaming in aboard air-conditioned buses, clutching laminated ID cards with maps of the settlement, wearing green bracelets bearing their names. Nearly all come from the most famous camp in post-quake Port-au-Prince: the Petionville Club golf course, home to 45,000 quake survivors, elements of the U.S. Army 82nd Airborne and a gaggle of Hollywood volunteers led by Sean Penn.
Aid workers lead the smiling tenants to their Chinese-made cylindrical tents, pointing out the floodlights, the police tent and where the 342 toilets and 24 showers are being built.
The plan is to stage about 6,000 people here along the 50-acre (20-hectare) ''Sector 4'' as the rainy season gets under way, even while U.N. trucks, U.S. Navy engineers and aid groups continue construction. Then they will start building sturdier shelters of wood, plastic and metal in adjacent Sectors 2 and 3.
There's no word yet on what will be built in Sector 1, but locals are expecting some major development. Concrete homes and stores are also being built around the new camp.
Manushka Lindor, 23, is among the lucky. She sat in the shady tent with her 3-year-old son, Peterson St. Louis Jr., who squealed ''Vroom! Vroom!'' as the big construction trucks went by. Just a few hours after arrival, she was already planning to stay.
''I don't have anywhere else to live. If they come here and build a house I can rent, I'd be very satisfied,'' she said.
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