From the website:
The most widely used military helicopter, the Bell UH-1 series Iroquois, better known as the "Huey", began arriving in Vietnam in 1963. Before the end of the conflict, more than 5,000 of these versatile aircraft were introduced into Southeast Asia. "Hueys" were used for MedEvac, command and control, and air assault; to transport personnel and materiel; and as gunships.
UH-1D, were nicknamed ‘Slicks' because they didn't have all of the encumbrances of the gunships. While the UH-1D was rated for two crew and twelve passengers, in Vietnam the crew was increased to include a crew chief and a door gunner.
The number of passengers was further decreased by the thinner air of the ‘hot and high' conditions of the Vietnamese highlands, with even six passengers proving a heavy load in the worst conditions.
Despite all the difficulties, the chopper pilots prided themselves in getting their passengers in, even under fire, and evacuating any wounded.
It was dangerous work and many helicopters came home riddled with bullet holes and with wounded pilots and crew.