The huge white hospital ship anchored in the Port-au-Prince bay is impossible to miss, drawing Haitians from all over in hopes of a chance to see a doctor and get medical treatment.

The USNS Comfort, on its fourth mission in Haiti since 2009, is open for business until September 18 in one of the western hemisphere's poorest countries.

And as has happened during every previous visit, the city's residents are turning out in droves for care that is otherwise hard to come by for the average person here.

"Everyone wants to get checkups when the Americans come," said Joanne, who declined to give her last name. "Me, I came at 2:30 am and there was already a line."

The 30-year-old woman was finally received by an ophthalmologist Friday afternoon. Her vision had deteriorated in recent months, and she was afraid she might have glaucoma but didn't have the money to go to a Haitian doctor.

"It's very expensive to be seen by a specialist and since the general hospital is under construction, they can't see everyone," she said.

Haiti's biggest public hospital was half destroyed in a catastrophic earthquake in 2010 that leveled parts of the capital city Port-au-Prince, killing over 100,000 people.

The hospital's reconstruction, co-financed by France and the United States with contributions of $25 million each, began nearly two years ago but may not be completed until the second half of 2017.

For Port-au-Prince's poor, the arrival of the US hospital ship is therefore a precious opportunity to get free quality health care.

In their crisp uniforms, the US Navy officers bustle about receiving, sorting and rapidly creating medical records for the hundreds of patients who have massed outside the gates to the naval base.

General medicine, pediatrics, dentistry, ophthamology, orthopedics: For the next week, the personnel of the USNS Comfort will treat more than 600 people a day at the base.

An equal number of patients will be seen at a hospital in the city near the US embassy.

Where required, patients will be taken on board the imposing 100-bed ship, where medical personnel are planning to conduct more than 100 surgeries over the course of the mission.

- 'Hands reaching hands' -

"Each time the US Comfort arrives in Haiti, a ton of lives are saved, a ton of therapeutic procedures are performed, particularly surgical procedures that truly help to save lives," said Florence Duperval Guillaume, Haiti's health minister, at an official ceremony Friday launching the mission.

US ambassador Pamela White, who is near the end of her tenure here, said the Comfort's visits were more than "just an ordinary visit between friends: this is hands reaching hands, this is true exchange."

"We are dreaming of the day when the Comfort has no need to visit but until then we'll continue to support Haitians," she said.

In a country where the average life expectancy is only 62 years, access to quality health care is often beyond the reach of the country's poor majority. The situation is even more critical for families who live in out-of-the-way rural areas.

The Pan American Health Organization reports that in 2013 Haiti had only 3.5 health care professionals per 10,000 inhabitants, and nearly half worked in the Port-au-Prince metropolitan area.