Flash cloning

Flash Cloning is a medical process used by the UNSC whereby human body parts are cloned. This usually involves a sample of a subject's DNA, which is used to grow a clone of an organ or body part, which can then be transplanted into the subject where the DNA came from. This way, organ transplants are faster and easier and the rate of success is far higher. Usually, the organ is programmed to grow at an accelerated rate, then to cease rapid development when transplanted into the subject. Therefore, the organ grows quickly and functions properly and efficiently. Since the organ contains the same subject's DNA, there is no threat of rejection by the body.

Human cloning
While flash cloning entire humans is possible, it is by no means perfect, and unauthorized cloning is illegal. The crime is punished with harshly, and is considered to have a severity on par with that of modern day child pornography. The process involves the modification of a human embryo, which causes it to develop one hundred times faster than it would have naturally, and become something different than it normally would have. Many anomalies appear because flash clones are forced to develop too quickly -- they lack the muscle memory and socialization of their hosts. Those these problems can be corrected with intensive therapy, the worst flaw in flash-cloned humans -- compounding biological defects -- cannot be corrected. One to two months after cloning, a flash-clone will start to degenerate from metabolic instability as part of a process called "metabolic cascade failure". They inevitably die from various neurological and physiological diseases.

Flash-clones were used in the SPARTAN-II Program. After the Spartan candidates were abducted from their homes, they were replaced with flash-clones as part of a cover-up. The flash-clones quickly died natural (though certainly abnormal) deaths, drawing suspicion away from ONI. Internal criticism of the unethical use of flash-clones led to the use of orphaned children in the subsequent SPARTAN-III Program.

Usage for Creating AIs
As an actual human brain is required for the creation of a Smart AI, since a brain is essentially destroyed during the process of creating the AI, the brain of a deceased person is often used. However, Cortana was modeled after the brain of Dr. Catherine Halsey who was obviously still alive. By flash cloning Dr. Halsey's Brain they were able to provide a brain used for the mapping of the AI's "mind" with no undue effects to the donor.