As many as 150 people have died in a cholera outbreak in Haiti, in a rural area north of the capital Port-au-Prince, government and aid officials said Friday. It is the worst outbreak of disease since last January’s devastating earthquake, which killed 250,000 people and left another 1.5 million homeless.

Cholera is a bacterial infection spread through contact with contaminated water. It causes severe diarrhea and vomiting that can produce dehydration and death within a few hours. It is invariably a disease associated with extreme poverty, since simple rehydration with a serum made with salt and sugar is generally effective as a treatment.

Once the disease emerges, it can spread exponentially through contact with the feces of infected people. There is great concern that the cholera could become a full-blown epidemic if it reaches the crowded refugee tent cities around Port-au-Prince, where more than one million people live under conditions of extreme squalor, with little or no sanitation and little access to clean water.

Health Ministry director Gabriel Thimothe said laboratory tests confirmed that the illness is cholera. He said the worst-affected areas were Douin, Marchand Dessalines and Saint-Marc, all in the Artibonite Valley, about 50 miles north of the capital. He told reporters that local hospitals were “overwhelmed,” and that children and the elderly were the most affected.

Medical workers at St. Nicholas hospital in the seaside town of Saint-Marc told of horrific scenes, with hundreds of dehydrated patients laid out on blankets in a parking lot with IVs in their arms, awaiting further treatment.

Doctor Jean-Robert Pierre-Louis told the French news agency AFP, “We’re facing an outbreak of diarrhea which causes rapid death of patients of all ages. This has to do with the quality of water in the affected communities.” An aid worker visiting the hospital told CNN, “It’s terrible. There’s a great fear among the people.”

The outbreak began Tuesday in the region along the lower portion of the Artibonite River. While the area was not greatly damaged by the earthquake, thousands of quake refugees fled to the region and are living there either in tents or doubled-up with relatives. This comes after recent heavy rains spurred the banks of the Artibonite to overflow and flood the area.

There were conflicting reports from surviving patients about how they contracted the disease. Some said they had been drinking water from a public canal, some from the Artibonite River itself, others were using bottled water, which may have come from a contaminated source.

Despite Haiti’s dire poverty, there has not been a cholera epidemic reported there in decades. But there have been repeated cholera warnings since the earthquake, which destroyed much of the country’s already inadequate infrastructure and left 1.5 million people living in tents.

The president of Food for the Poor, Robin Mahfood, told Reuters news service, “We have been afraid of this since the earthquake.”

Haitian Medical Association, Claude Surena, said, “The concern is that it could go from one place to another place, and it could affect more people or move from one region to another one.”

President Rene Preval said his government was taking steps to ensure the disease did not spread further. There were calls for a “sanitary cordon”—effectively a quarantine—around the Artibonite Valley.

It was not clear how the regime would stop people from moving from the region where the infection is currently localized, especially in the period leading up to national elections, set for November 28, when many people are expected to travel to their former home towns to vote.

Press accounts Friday said that new cases of cholera had been reported on the island of La Gonâve, and the town of Archaie, closer to the capital, as well as the town of Croix-des-Bouquet.

Haitian Health Minister Alex Larsen announced Friday that tests by the World Health Organization had confirmed that “the outbreak is of the 01 strain of cholera which is the most deadly and is responsible for most of the outbreaks around the world.”

The American Red Cross issued a statement of concern, noting, “This is not an area directly affected by the earthquake of January 12, but we are concerned about the speed with which this outbreak has spread.”

The poor conditions in Port-au-Prince are demonstrated by the events of last weekend, when heavy rains created mudslides that killed at least 12 people, including eight in one neighborhood on the western edge of the capital.