The number of cholera cases is rising in parts of Haiti hit by heavy rains earlier this week.
Alain Legarnec, mission chief for the French aid group Doctors of the World, said a clinic in the south-western town of Jeremie treated 77 people for cholera in recent days.
That is a five-fold increase from last week and was most likely caused by rising river levels, he said.
Haiti and its Caribbean neighbours were hit by a deadly storm on Monday that flooded towns and destroyed houses. The Haitian capital and southern part of the country were worst hit.
The storm system has dumped up to seven inches of rain in Port-au-Prince since May 30, according to Bob Smerbeck, a senior meteorologist for AccuWeather, a company of forecasters based in State College, Pennsylvania.
The mountains in Haiti could have received double that amount of rainfall, Mr Smerbeck added.
The showers contributed to widespread flooding and mudslides. The death toll in Haiti was put at 28, Marie Alta Jean-Baptiste, director of Haiti's Civil Protection Department, said. But that number could rise as families search for missing loved ones. Ms Jean-Baptiste said six people are still missing.
The storm came only a few days after international aid group Oxfam said its staff saw a spike in the number of cases in Carrefour, a densely populated area west of Port-au-Prince. Oxfam said its aid workers were treating more than 300 new cases a day, which was more than three times what they saw back when the disease peaked in the autumn.
Since then, the number of cholera cases in Carrefour has dwindled but flared up elsewhere in the Port-au-Prince area, according to Sylvain Groulx, Haiti's chief of mission for Doctors Without Borders.
"In Port-au-Prince, what we're seeing is the outbreak is ongoing and spreading," Mr Groulx said. "Whenever there's a rainy season... it often has consequences on these epidemics."
In the first two days of this week, two Doctors Without Borders clinics treated 444 cases in Port-au-Prince.
The week before, the aid group treated more than 2,500 patients in the capital, compared to the lowest number of cases in one week - 302 cases, in April. Doctors Without Borders treated 4,645 people in one week at the height of the epidemic in November.
Cholera, a bacteria transmitted by water, has sickened more than 330,000 people and killed nearly 5,400 people since the Haiti outbreak began in October.