Five cholera patients have been reported in Port-au-Prince, where more than a million survivors of the massive earthquake in January are in living in dismal conditions in tent camps.
If cholera reaches the camps, the death toll could be devastating.
"This really has been our worst fear, that these camps will be struck with the disease," said Margaret Aguirre, communications director of the International Medical Corps. "It's something we have been planning for over the last nine months."
A team of doctors and nurses has been in Haiti since the earthquake to provide medical care and education to Haitians about water, sanitation and hygiene, all keys to preventing a major medical disaster.
Cholera can cause vomiting and diarrhea so severe, people can die from dehydration within hours.
The IMC has a team of about 300 aid workers in Haiti.
"We're actually assessing whether we need to activating existing networks," Aguirre said.