The International Committee of the Red Cross told CNN 30 cases have been reported in the last three days and seven inmates have died.
The prison has 2,000 inmates.
"It's overcrowded and it can spread very, very fast," said ICRC spokeswoman Olga Miltcheva.
Spokesman Marcal Izard told the Canadian Broadcasting Corp. the Red Cross is working with the Haitian government to "make this prison safe."
Doctors Without Borders, however, criticized the response as inadequate.
"More actors are needed to treat the sick and implement preventative actions, especially as cases increase dramatically across the country," said Stefano Zannini, the group's chief in Haiti. "There is no time left for meetings and debate."
Mark Ward of the U.S. Agency for International Development said it has "a lot of confidence" in the Haitian government's actions.
The United States is focusing Haiti aid on prevention, he said, citing the need for clean drinking water, chlorine for the water supply, oral rehydration salts, education for prevention and funds to expand clinics.