Solid Snake
06
As an artificially engineered human, Solid Snake's lifespan was predesignated through telomere and gene modification. As part of the Les Enfants Terribles Project, he was engineered to be short-lived to prevent enemy forces from copying his genetic code.
Solid Snake's accelerated aging also brought about a change in the FOXDIE virus within him.
Damage to the victim targeting receptor was on the verge of yielding a mutant form of FOXDIE capable of killing indiscriminately. The time limit until that mutant form arose was three months. After that period, Snake would become a walking weapon of mass destruction. Furthermore, another new FOXDIE strain was discovered within his body.
But ironically, the new FOXDIE virus, injected into Snake by Drebin, eliminated the mutant FOXDIE. It was by becoming a vector for the new FOXDIE strain that Snake avoided becoming a weapon of mass destruction.