(CNN) -- Battered by a devastating earthquake, left for nearly a year without real homes, promised aid that failed to arrive, the people of Haiti now face a new killer, and the littlest children are among the hardest hit.

"The heart-wrenching piece of all of this is the children, who we have seen are suffering the most," aid worker Roseann Dennery said from the desperately poor Caribbean island nation.

She doesn't flinch in describing the effects of cholera; the water-borne disease has claimed at least 259 lives so far and is spreading quickly.

Children "are coming in with hard-to-control diarrhea and vomiting. Their little lives are frail, weakened. And so scared," she said in an iReport.

"Robens Jeune and his 2-year-old son came into the clinic. His little boy looked up with wide eyes and sat on the cot, scared and suffering. We started an IV and sat with him and his father to quiet his crying," said Dennery, who is with a Christian aid organization called Samaritan's Purse.

"Today, he just started throwing up," the boy's father told Dennery as he placed his hand on his son, Frantzley.

"I was on the way to the Saint-Marc hospital and someone told me that there was a clinic here, closer to home. So we came. And he has perked up, he is feeling better. I am hopeful he can live through this," Jeune told the aid worker at a rehydration clinic her group set up in Villard, near the center of the outbreak.

In theory, cholera should not be hard to control or to treat -- which is why aid organizations are racing to tell Haitians how to avoid it.

"First of all, drink clean water -- bottled, treated or boiled water," said Abdikadir Hassan, head of the Mercy Corps office in Mirabalais, downriver from the center of the outbreak.

The aid agency is telling people to "wash their hands every time they do something -- go to the toilet, eat. If you have enough water, wash the food before you eat. We're trying to give them soap and water treatments."

Mercy Corps is not waiting for Haitians to come to them for help.

"We put speakers on a van," Hassan said by phone from the town of Mirabalais. "We're going out to the community, we're at the market, we're at the schools."

They don't get to everyone in time.

When Hassan got to the local prison, he found it had 50 cases of cholera, of whom two had already died.

Next he went to the local hospital, where there were 800 cases, of whom 10 have died, he said.

"It started with children and then adults," he said. "In the past few days we have seen more children."

Mirabalais is a textbook case of how shock waves have rippled out from the earthquake since it struck in January.

It's on the central plateau in the center of Haiti, Hassan explained.

"It was not affected by the earthquake, but it received families that were," he said -- about 16,000 people made homeless came to stay with family members in the area.

More than nine months later, about half those people are still there, known officially as internally displaced persons and living with host families and relatives, Hassan said.

"We don't have IDP (internally displaced person) camps where I am," he said

"They are staying in villages, they are staying in towns -- but they are staying in... very small houses with eight or 10 people, houses that normally have four or five people," he said.

Those are exactly the kind of conditions that make it easy for disease to spread, and that's what worries aid workers.

"It would be irresponsible to plan for anything but a considerably wider outbreak," said Nigel Fisher, U.N. humanitarian coordinator in Haiti.