Slumped in a wheelchair in the hospital yard, Dieuson Dorvil had just begun spelling his name when a spasm gripped his chest and he turned away to vomit.

His mother, Isma, held him as he shuddered and heaved, filling a bucket at his feet. It seemed impossible that such an emaciated figure could produce so much but the disease was sucking the fluid from his body.

Around the yard and inside the wards babies and children, men and women, lay curled and motionless, their stares glassy. The only sound was the occasional retch.

Every so often a stretcher with a figure half shrouded in a white, soiled sheet was wheeled to the morgue. They bounced over the gravel and potholes, giving the impression the corpses were trembling.

This is what Haiti's cholera epidemic looks like. It turns people into vomit- and faeces-producing spectres. It is filthy, smelly and a horrible way to die.

The official death toll has approached 800, with 11,000 taken to hospital, and there is dread that this is just the beginning. What started a month ago as isolated diarrhoea cases in the rural Artibonite valley has turned into a nationwide cholera outbreak, with more than 1,000 new cases daily.

People in towns and villages around Gonaïves, in the north, are burying victims in mass graves, according to aid workers and photographers, and suspected cases have been detected in the south and in the Dominican Republic, which shares this Caribbean island with Haiti.

The biggest concern is the capital, Port-au-Prince. Still in ruins from January's earthquake, it is a metropolis of rubble and tents and open sewers that condemn most of its 3 million inhabitants to unsanitary, crowded conditions.

"All of the hospitals in Port-au-Prince are overflowing with patients and we're seeing seven times the total amount of cases we had three days ago," said Stefano Zannini, Médecins sans Frontières' head of mission in Haiti. "I can easily see this situation deteriorating to the point where patients are lying in the street, waiting for treatment."

Outside the gate of the cholera clinic at the city's main hospital lay a teenage girl in a peach T-shirt and khaki shorts. She appeared unconscious, her head lolling at an awkward angle, brown stains down her left leg. No one knew her name or how she had come to be there.

The clinic, an improvised centre with tarpaulin, was full. Before entering, visitors wiped their feet on a ratty yellow cloth sprayed with chlorine. The yard had about 10 silent patients attached to IV drips. The only light was a sliver of moon.

"The doctor hasn't see him yet, I don't know what's going to happen," said Isma, 42, after another bout of vomiting from Dieuson. Eyes sunken, the 24-year-old student drifted in and out of the interview. "It feels like there's something inside, squeezing," he said.

Isma said she was a restaurant cook but that business had evaporated. "No customers. Everyone's afraid of getting the infection." The family lived near the Cité Soleil slum where hundreds, possibly thousands, have been stricken. "My son is the first in our family to get sick," she said, feeling his forehead. "We don't know how it happened."

The clinic's wheelchairs have white plastic seats cut from garden furniture, lending an incongruous jauntiness to the wretchedness. When a patient missed the bucket, or became lathered in slickness, a man with a white mask and blue overalls sprayed chlorine.

In the wards men and women lay on wooden frames with a round hole cut in the middle and a red MSF bucket beneath. "We call it the Calcutta stretcher," said a young Haitian doctor, the only one on duty. "I don't know if we can keep up. It's spreading so fast." From a handful of patients earlier in the week there were now more than 70 daily.

The doctor declined to give his name. With elections due on 28 November, and mounting anger at the government's response to the crisis, the epidemic is politically sensitive.

The UN is also defensive over speculation Nepalese peacekeepers inadvertently introduced the disease, which closely resembles a south Asian variant, into the Arbonite river. Now that it is in the water supply it will last months or years, according to epidemiologists.

The UN appealed today for $163m to respond to the crisis, which it said could infect up to 200,000 people. "Cases are expected to appear in a burst of epidemics that will happen suddenly in different parts of the country," Elisabeth Byrs, of the Office for the Co-ordination of Humanitarian Affairs, told a news briefing in Geneva.

With timely treatment most cholera victims survive, but poverty, fear and a destroyed infrastructure mean many Haitians seek help late, if at all. Records at Port-au-Prince's main clinic showed 24-year-old Marie Renée arrived at Rue Mgr Guilloux at 4.30pm. By 4.33pm she was dead.

A few hours later it was the turn of an elderly man. Evelt Pierre Juste, bearer of the dead, rolled the body, his 16th of the day, up the hill to the morgue. He went fast, lest other patients' eyes lingered on the shrivelled figure. He entered a dank passageway and tipped the body on to a cement floor beside the curled, naked form of Marie Renée. The fridge was locked for the night so the two strangers would lie side by side until morning.

At the morgue entrance a watchman, Wilzor, huddled by a radio listening to upbeat Compas music. He needed to listen to something cheerful, he said. "I ask myself how bad it's going to get. But only Jesus and the disease can answer that."