Caribbean islands battered by Hurricane Irma now face a mighty task: preventing the rampant spread of disease as hospitals and clinics struggle to function and microbe-spreading mosquitoes and rats multiply.

Ciro Ugarte, health emergencies director for the Pan American Health Organization coordinating the Caribbean response, spelled out the scale of the trials ahead.

- Challenge No. 1 -

With health facilities mangled by the vicious storm, the first priority will be to treat injured people wherever they are, setting broken bones and stitching up wounds.

Medical volunteers have been dispatched from several countries, with the aid of military helicopters from French, Dutch, American and British armies, to help stabilize patients, said Ugarte.

"Then we will have to evacuate those people," he told the AFP news agency.

"We planned to do that evacuation by road (but) it will not be possible in several islands. So we are planning to evacuate by sea and also by helicopter in some places."

The numbers are not yet known, but "we are planning in this case for the worst."

Some medical supplies will have survived in specially-reinforced warehouses, but even so there would be "just enough for the first hours of the response," Ugarte said by phone from Washington.

"The rest has to be complemented from overseas," he added. "We are also sending supplies and medicine from our humanitarian warehouse that is based in Panama."

- Challenge No. 2 -

Within 48 hours, infectious respiratory diseases and diarrhea may start becoming a problem.

"That effect will be almost immediate because of the lack of running water, potable water, and also because of the loss of houses and shelter for the people," said Ugarte.

With limited access to medicines, bundled into large groups in shelters, people will see infectious diseases such as colds and flu spread fast and progress more easily from mild to severe, potentially deadly -- especially for the young and the elderly.

With clean water supplies disrupted, cholera -- an intestinal infection spread by faeces-tainted water -- is a real concern.

"After that, we will see other diseases that are vector-borne diseases," said Ugarte. "That will happen in the next two weeks or so."

These included Zika, dengue and chikungunya -- all viruses spread by mosquitoes already in circulation in the Caribbean.

"All the water, because of the flooding, will provide places for mosquito breeding," said Ugarte. "There will be a huge increase of mosquitoes and that will increase the risk for those diseases."

There will also be an increased risk for a different kind of mosquito, which transmits malaria, to take root.

Another threat is rats, which tend to flock to standing water and human food stocks. They notably carry leptospirosis, a fever-causing bacterial disease which can cause brain inflammation in humans.

- Challenge No. 3 -

It may take months to get flood-damaged hospitals back on line, and years to reconstruct entire island health services, said Ugarte.

"The effect is devastating for those countries. It is really, really bad. It is something that has not been seen in decades."

Health services will not recover fast enough to cope with the demand, he warned.

"This is not only related to diseases, injuries and vector-borne diseases" in the aftermath of the storm.

"It is also related with other health needs, for example children and mothers need to be checked and need to be vaccinated, and they may not because the health services availability may not be there."

People with chronic diseases such as diabetes or high blood pressure may miss out on the treatment they need.

"Recovery certainly will last for years," said Ugarte.