Doctor

A doctor, or "Physician", is a person, or animal, namely Tony Tony Chopper, who practices human biological medicine.

One Piece
Doctors are relatively essential members of any crew, as a crew, whether it be Pirate, Marine or any crew in between, needs someone good with medicine and health care in order for the crew survive to harshness of the sea. Doctors are usually required to heal wounds from battles, or prepare medicine for sick members.

In terms of Luffy's crew the issue of having a doctor was first brought up in the Arlong arc, when the doctor of the town, Dr. Nako, said they really needed a doctor, after patching up Zoro's wounds. Luffy said it was a good idea, but they needed to find a musician first.

After the Drum Island arc, Tony Tony Chopper joined the crew as the ship's doctor (although, Luffy originally wanted him because he was a "cool monster that can change into seven forms"; Sanji joked saying he was the "emergency food supply"). Since then, Chopper has fulfilled his duty as a doctor, patching up wounds from battle, and using his knowledge of medicine to the crew's advantage.

Notable Doctors

 * Tony Tony Chopper (Straw Hat Pirates)
 * Dr. Hiruluk
 * Dr. Kureha
 * Isshi-20
 * Kaya
 * Dr. Nako
 * Dr. Ooh
 * Dr. Potsun
 * Doc Q  (Blackbeard pirate)
 * Crocus (Jolly Roger Pirates)
 * Dr. Hogback
 * Trafalgar Law