For the residents of Mocajuba Island, dirt-poor folk who sleep in wooden shacks under the Amazon's vast canopy of green, the once-a-year visit by the hospital ship is an occasion similar to Christmas.

They greet the two-level white vessel and its team of volunteer medical staff with the enthusiasm and attire reserved for special occasions.

Once on board, they eagerly push forward, without exception, to be treated for the variety of ills accumulated in a part of the world almost untouched by civilization.

This is the mission of the Light of the Amazon, a floating hospital run as a charity program since 1962 to improve the physical health of the riverside denizens in Brazil's remotest region.

The ailments suffered by the community of 250 people, located four hours upriver from the northern city of Belem, attest to the harshness of jungle life and ignorance about hygiene.

"Intestinal parasites are very common, due to the untreated water that they usually use. Even if they treat the water, the kids go swimming in the river and swallow water from the river," says one of the doctors, Maria-Auxiliadora Martins.

She is treating a young girl with an ear infection and a stomach parasite. Most likely she also has the most common ailment seen by the staff: decaying teeth.

The onboard dental surgery works for such long hours treating the villagers that the two dental student nurses working the drill complain of shoulder pain afterwards.