Health authorities have detected the first cases of cholera in the capital of Haiti, as the death toll from the outbreak in the rest of the country passed 250, raising fears that the epidemic was spreading through the city's camps and slums.
The government and aid agencies raced to set up treatment centres in rural areas north of Port-au-Prince, where the epidemic started last week, and across the capital.
"Cholera centres are going up as we speak. We are moving as fast as possible without slipping into panic," said Melody Munz, the environmental health co-ordinator for the International Rescue Committee (IRC). "It's very worrying. This can spread very rapidly."
Haiti's first cholera outbreak in a century has killed at least 253 people and infected more than 3,015 with the virulent diarrhoeal disease, overwhelming some clinics and causing the country's worst public health crisis since January's earthquake.
In a separate development, American seismologists warned that the Caribbean nation may be in line for another earthquake. The magnitude 7 quake which flattened much of Port-au-Prince in January may have been caused by an unseen fault, according to two papers in the journal Nature Geoscience, meaning the Enriquillo-Plantain Garden fault which was originally blamed could be building up pressure and remains a threat.
The only good news today was a suggestion that the worst of the cholera outbreak may be over. "We have registered a diminishing in numbers of deaths and of hospitalised people in the most critical areas," Gabriel Thimote, director-general of Haiti's health department, told a news conference. "The tendency is that it is stabilising, without us being able to say that we have reached a peak."
Authorities hoped to contain the epidemic in the Artibonite region, about 40 miles north of the capital, but over the weekend five cases were confirmed in Port-au-Prince.
The UN said the five were infected in Artibonite before travelling to the capital, meaning the city was not considered a new location of infection.
However there were other suspected cases in Croix-des-Bouquet, just outside the city, and in Carrefour and Petionville, neighbourhoods inside the city.
The speed with which cholera centres were being set up in Port-au-Prince – tonight there were 12 – showed authorities were bracing for the worst.
Cholera is an intestinal infection caused by bacteria transmitted through contaminated food or water. It causes rapid and severe dehydration which can kill within hours, especially if the person is very young, old or weak.
Authorities urged people to wash their hands, not to eat raw vegetables, to boil all food and drinking water, and to avoid bathing in or drinking from rivers. Cholera can be treated by drinking clean water mixed with salt and sugar.
Squalid conditions in the rubble, tents and slums which are home to much of the city's population are ideal breeding grounds for the disease – soap, clean water and latrines are all scarce.
The Haiti Epidemic Advisory System, a biosurveillance project operated by Praecipio International, a non-profit group, rated the outbreak on its impact scale as a category five, defined as an "infectious disease event associated with disaster indicators".
Given the population's lack of protective immunity the outbreak was likely to spread widely, said Jon Andrus, deputy director of the Pan American Health Organisation.
The earthquake killed about 300,000 people, left another 1.5 million homeless and destroyed infrastructure which has only partially been replaced by the UN and battalions of NGOs.
Even before the disaster, sanitation was appalling but with so many people homeless the level of "open defecation" had spiked, said Munz. Aid groups were distributing soap and chlorine for water but Haitians must take responsibility for their own hygiene measures, she said.
"Just one gram of faeces contains about 1m bacteria so you don't need very much to get sick. It's very easy to be exposed."
Authorities worried that two national holidays next week would spread the disease as people travelled to visit friends and family.
The Artibonite river, which drains central Haiti, is suspected to have infected people who use it to bathe and drink. Heavy rains, which can turn landscape to mud in just minutes, have made containment harder.
Munz hoped stigma would not deter those infected from seeking treatment at cholera centres, which have just one entry and exit, rather than regular hospitals. "We are treating people in the most dignified way possible. We are trying to keep the stigma away but we can't control what people think."
What is cholera?
An intestinal infection caused by the bacterium Vibrio cholerae. It induces diarrhea, vomiting, fever and abdominal pains and rapidly dehydrates the body.
How is it spread?
Via contaminated food or water, which is especially common in slums and camps with poor sanitation. Global warming has been blamed for breeding cholera-friendly conditions. About 75% of people infected with cholera do not have symptoms but can spread it through faeces.
How do you prevent it?
Drink only clean water, avoid raw vegetables, wash hands with soap. The cholera vaccine is not 100% effective, but the World Health Organisation (WHO) backs mass immunisation campaigns in emergencies.
How do you treat it?
Most of those infected can be cured with oral rehydration tablets or clean water mixed with salt and sugar. Extreme cases need intravenous drips.