The Comfort is not manned and ready for sea. She is, instead, kept in a state of "reduced readiness", requiring that she could put to sea within five days of being so ordered. She has two crews, much like an aircraft carrier. One crew is the ship's company. These are the people who operate and sail the ship. Then there is the medical company. These are mostly reservists who practice medicine in the civilian world. It takes time to get this entire compliment to the ship and ready to go.
That go date for the mission to Haiti is tomorrow.
From the opening of her Wiki page:
USNS Comfort (T-AH-20) is the third United States Navy ship to bear the name Comfort, and the second Mercy Class Hospital Ship to join the navy fleet. In accordance with the Geneva Conventions, USNS Comfort and her crew do not carry any ordnance and firing on the Comfort is considered a war crime.
Like her sister ship USNS Mercy (T-AH-19), Comfort was built as an oil tanker in 1976 by the National Steel and Shipbuilding Company. Her original name was SS Rose City and she was launched from San Diego, California.
Her career as an oil tanker ended when she was delivered to the Navy on December 1, 1987. Now, as a hospital ship, Comfort's new duties include providing emergency, on-site care for U.S. combatant forces deployed in war or other operations. Operated by the Military Sealift Command, Comfort provides rapid, flexible, and mobile medical and surgical services to support Marine Corps Air/Ground Task Forces deployed ashore, Army and Air Force units deployed ashore, and naval amphibious task forces and battle forces afloat. Secondarily, she provides mobile surgical hospital service for use by appropriate US Government agencies in disaster or humanitarian relief or limited humanitarian care incident to these missions or peacetime military operations.
When not actively deployed, Comfort is kept in a state of reduced operations in Baltimore harbor. She has been used many times over the years and has been ready to ship out of Baltimore with 5 days' notice.
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More here:
http://en.wikipedia.org/wiki/USNS_Comfort_(T-AH-20)