Efforts to rebuild the poverty-stricken island were led by the United Nations. But in a cruel twist, U.N. soldiers sent there to help are thought to have inadvertently started a cholera epidemic one year later.

Now a lawsuit is being brought by more than 5,000 Haitians.

We have this report by Inigo Gilmore of independent television news.

INIGO GILMORE: The Artibonite River is in many ways Haiti's life source. For generations, people have come here to bathe. It had always provided a natural and safe source of drinking water, too -- that is, until it was poisoned with cholera, just over three years ago. People around here started dying.

WOMAN (through interpreter): This is where we take water to wash our clothes, to shower, to drink. And the U.N. is up there dumping their bathroom waste into the water. We got infected from the water.

INIGO GILMORE: Soldiers stationed at this United Nations base perched by the river in the town of Mirebalais were accused of being the source of the cholera outbreak.

In October 2010, it was alleged that dark liquid from an overflowing septic tank was spewing from the base into the river.

WOMAN (through interpreter): My daughter got cholera when she was 2 years old, and, recently, she got sick again. She spent three days in hospital. She was much bigger than this. She's lost a lot of weight.

INIGO GILMORE: Three years on, there's been nearly 700,000 cholera cases. Now this insidious disease is growing more deadly.

At this Medecins Sans Frontieres hospital in Leogane, we met countless young families in distress.

WOMAN(through interpreter): She wasjust crying. She was crying for three days, so we drove her here. She had diarrhea in the car. When we got here, they ran some tests, and it revealed she had cholera.

INIGO GILMORE: The doctors are acutely worried about her child, Alchena. She's malnourished and fungus is spreading inside her mouth.

Another sick baby, Ruji, is just six months old. Ravaged by their harsh living conditions, young and old are succumbing quickly to cholera, which should be easy to treat. But the number of clinics and doctors here is actually decreasing, as Haiti drops off the international agenda and aid budgets are slashed.

DR. KENNETH LAVELLE,Doctors Without Borders: There are less and less treatment centers, you know. There is less and less preventive activities. So, as the organizations, as the government have disengaged from the day-to -day management of cholera, the number of deaths is increasing.

This is the only facility where they can come. Everyone else, including the Ministry of Health, they're not present. They're not engaged in this medical activity, which is absolutely unacceptable.

INIGO GILMORE: Haiti had never had a recorded case of cholera before 2010. Proof of its source is not definitive, but the scientific evidence from international and local experts has been stacking up.

DR. JEAN ANDRE VICTOR,Haitian Association of Environmental Rights (through interpreter): Scientifically, you can't be 100 percent sure. But the waste from the base was being dumped into the river. And the first victims were drinking water from the river.

The soldiers at the base came from Nepal, a country where cholera exists. The bacteria we identified matches the one from Nepal. All this cannot just be coincidence.

INIGO GILMORE: It's nearly 10 years since the United Nations Stabilization Mission in Haiti was launched. It's the third largest peacekeeping operation in the world, even though Haiti has not been at war.

The U.N. mission has a fractious relationship with Haitians. Its force is accused in dozens of rape and sexual abuse cases, and now for bringing cholera. The epidemic is now reaching some of the most remote corners. Surrounded by rice paddy fields, this village lies more than two hours downstream from the U.N. base in Mirebalais. But, even here, cholera has taken a heavy toll.

MAN (through interpreter): This is the water we used to drink from. We got cholera from this water.

INIGO GILMORE: Villagers step forward to tell us about the loss of their loved ones.

MAN (through interpreter): I lost my child. And I had cholera myself. When I lost my child, I thought he'd been poisoned. I took him to a traditional healer. Before we could do anything, the child was dead.

INIGO GILMORE: This old man lost his brother, a cousin, and his two children. He seemed shell-shocked, wondering aloud who would now look after him.

They took me to a nearby cemetery. And in the undergrowth, blue plastic sheeting was clearly visible. They told me how they wrapped the bodies in plastic and buried them hurriedly in unmarked graves, fearing the spread of contagion. There's no dignity in death around here. A human bone was lying near one grave.

First, there was sorrow. Now there's real anger.

MAN (through interpreter): The United Nations must be held accountable. We lost a lot. They should compensate us and they should do it right away.

INIGO GILMORE: Their cause has been taken up by Haiti's leading human rights lawyer, who is seeking compensation from the U.N. for over 5,000 Haitian victims, whose plight, he says, is being ignored.

MARIO JOSEPH,Bureau des Avocats Internationaux (through interpreter): The recognition of human rights for rich people, human rights for poor people, we can't accept that, because United Nations is an organization for the world.

INIGO GILMORE: Mario Joseph has now launched a lawsuit in New York's federal court to challenge the U.N.'s claims of immunity from prosecution. But over at the U.N. headquarters in Port-au-Prince, there's a refusal to even discuss the issue.

Why didn't the United Nations committee just admit it's responsible for the outbreak of cholera in Haiti?

SOPHIE DE CAEN,U.N. Humanitarian Coordinator in Haiti: Well, unfortunately, I can't comment on that particular side of the issue.

INIGO GILMORE: Why?

SOPHIE DE CAEN:Because we're not supposed to be commenting on issues that are being considered by the legal office in our -- in our headquarters.

INIGO GILMORE: But it's not just a legal matter. There's a moral matter here as well. People are appalled by what's happened. They blame the U.N.

SOPHIE DE CAEN:As I said, I'm sorry, but I can't -- I can't comment on it. I think what's more important is how to deal with it here and now.

MARIO JOSEPH: The United Nations promotes the due process, the rule of law, the human right. They need to give Haitian people a day in court.

INIGO GILMORE: But as he seeks his day in court against the U.N., it seems he won't be getting much help from his own Health Ministry, which is clearly reluctant to take on the world body.

MARIE GUIRLAINE RAYMOND,Haitian Health Ministry (through interpreter): We support the Haitian people who are victims of cholera.

INIGO GILMORE: You said you support victims of cholera. Do you support the claim of 5,000 against the United Nations?

MARIE GUIRLAINE RAYMOND(through interpreter): We support the Haitian people.

INIGO GILMORE: Can you answer the question? Do you support this claim, please?

MARIE GUIRLAINE RAYMOND (through interpreter):Do you answer my -- do you Do you hear my answer? You are the level of the Health Ministry.

INIGO GILMORE: Suddenly, she'd had enough. She headed for the door, jumped in her car, and drove off.

The Haitian government says it's working with the U.N. on a 10-year plan to rid Haiti of cholera. But the aid agency trying to hold back a disease that's already claimed over 8,000 Haitian lives says it's an emergency right now.

DR. KENNETH LAVELLE:So who's going to treat these patients, yes? This problem is not going away, yes? An eradication plan over 10 years is a great idea, a great initiative, but it doesn't address the needs of the Haitian people today.

INIGO GILMORE: Four years ago, the world responded to Haiti's massive earthquake with promises to rebuild the country and make it better than before. Four years on, many pledges still remain unfulfilled, and the world body stands accused of heaping more misery on this ravaged people.