The WHO said that 800,000 people have been affected by the four major storms which Haitian authorities say have left at least 326 people dead.
Just over half of those affected (52 percent) are women and 36 percent children.
"Elderly people and pregnant women are among the most vulnerable needing care, as are patients with chronic disease and those being treated for HIV and tuberculosis," the WHO said in a statement.
The series of storms -- two tropical storms and two hurricanes, including Ike which grazed Haiti's waterlogged northwest last weekend -- "is the largest catastrophe to strike the country in several years," Interior Minister Paul-Antoine Bien-Aime said Thursday at a news conference.
In addition to the dead and missing, the toll includes 190 people wounded, 170,000 families stricken by the storm, and 151,000 refugees in storm shelters, according to authorities.
Flooding has severely affected health facilities such as hospitals in Gonaives, Port de Paix and Les Cayes, and drugs such as insulin and anaesthetics are in short supply, the organisation warned.
"The situation is particularly serious in Gonaives, where parts of the city remain under water, people have taken refuge in shelters and there is no access to some communities outside of the city," said Henriette Chamouillet, the WHO's representative to Haiti.
One million dollars are required to coordinate the health sector response and the WHO is sending staff from its Geneva headquarters to provide logistics support, including ensuring medical supplies are delivered to communities cut off by flood waters.
The remaining 3.2 million dollars is destined for urgent health projects run by WHO partners such as UNICEF.