International health officials told CNN the outbreak, which has sickened 57,000 people so far, is likely to top earlier predictions of 200,000 cases in the coming months.
"Having seen how the bacteria is behaving in this environment with these people, having seen just how poor and how hungry the people are, we know we have to revise our numbers up," said Nyka Alexander of the World Health Organization.
Since cholera had not hit the island in more than a century, the population has no immunity and Haitian medical workers lack experience treating it, international health workers said.
WHO said it has distributed enough rehydration salts for 60,000 people and enough intravenous fluids to treat 20,000 severe cases.
But last week the United Nations said less than 10 percent of the emergency $164 million it asked for had come through.
There are two major shortages, Alexander said, doctors and nurses trained to treat cholera and "trucks to carry dead bodies."