When the U.S. Air Force retired the C-141 transport in 2004, there was a problem in finding an aircraft to take over CCAT (Critical Care Air Transport) missions. The C-17 was expected to take over this, along with most other C-141 missions. But the C-17 was in very heavy demand after September 11, 2001, and the air force scrambled to find an alternative aircraft for getting badly hurt troops to specialized medical care in distant locations (like from Afghanistan to the United States).
In Iraq, it was soon found possible to use KC-135 aerial tankers to carry CCAT teams and their patients. The huge demand for air transportation to support Iraq operations allowed the KC-135 tanker to operate more frequently in its secondary role as a personnel and cargo transport. The KC-135 always had space for cargo or passengers, but has rarely used in this role because most of its lifting capacity was taken up with fuel for aerial refueling. That changed about seven years ago.
Every KC-135 can carry a combination of 40 tons of cargo or 37 passengers. In Iraq and Afghanistan, it became common for KC-135s to move wounded troops to hospitals in Germany or the United States. Previously, the C-141 and C-9 was used for this, but the C-9 aircraft was retired in 2003. Many of the other aircraft used for this, like C-130s, also have to make refueling stops to get to the United States. A KC-135 can do it non-stop, which is easier on the patients. So KC-135s coming back from a tour in Iraq or Afghanistan, will often carry patients, and a CCAT team.
The KC-135s used for CCAT eventually had modifications made, like adding better lighting (usually LEDs) to make it easier to monitor, or work on, the patients. These special LED lighting fixtures are now made available as kits, that can quickly be installed in the cargo space of a KC-135. Other changes needed in the KC-135 were the oxygen systems and attachment points on the hull for medical gear and stretchers. Most of the KC-135s used for CCAT were from reserve (usually National Guard) units, and the maintainers of these aircraft usually had the time to rig their KC-135s for CATT operations after they were activated and before they headed overseas.
Since combat casualties were first moved by airplane during World War II, the rule was that the patient had to first be stabilized. That meant that patients in need of the most complex intensive care (specialist surgeons, special equipment) would often die because they needed that special care to be stabilized in the first place. In the 1990s, the U.S. Air Force came up with the CCATT (Critical Care Air Transport Team), which consists of a doctor, a critical care nurse, a medical technician and a lot of portable emergency room equipment. Like computer equipment in general, medical gear has become smaller and more efficient. So the CCAT team can basically carry their portable emergency room onto any transport aircraft that is carrying one or more patients in need of specialist care. The medical personnel take care of most medical emergencies while in flight, and the concept has already saved the lives of American troops badly wounded in places like Afghanistan, Iraq and the Philippines. The three members of the CCAT teams undergo two weeks of training on how to operate an ICU (Intensive Care Unit) in the sky.
Iraq had less need for CCATT because it was a large operation with extensive medical facilities in the area. But even in Iraq, there is sometimes a need to get very badly wounded troops to specialist facilities in Europe or the United States. Since most of the fighting has shifted to Afghanistan, an area with much fewer medical facilities, the KC-135 CCATTs have been a lot busier.