Flash cloning

Flash cloning is a medical process used by the humans to clone body parts. The process uses a sample of a human's DNA to grow a cloned organ, which can then be transplanted into the subject that contributed the DNA sample. Usually, the organ is programmed to grow at an accelerated rate and cease rapid development when transplanted into the subject. This makes the method viable in urgent situations, where patients can't afford to spend years waiting for a normal organ to develop. Since the transplanted organ's DNA matches that of the recipient, the risk of transplant rejection is completely eliminated.

Human cloning
While flash cloning entire humans is possible, it is by no means perfect, and unauthorized cloning is illegal. The crime is punished 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. Many anomalies appear because flash clones are forced to develop too quickly -- they lack the muscle memory and socialization of their hosts. Though 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 AI
Because humanity is a Tier 3 civilization and the creation of a "smart" AI requires the destruction of a human brain, the brain of a deceased person is often used. There is one known exception in the Halo Universe however: Cortana was modeled after a flash clone of Dr. Catherine Halsey's brain. The use of a clone allowed the UNSC to provide a brain used for the mapping of the AI's "mind" with no undue effects on the donor.