While the Clones were an excellent fighting force, they were mere copies of an ordinary human body (in fact, in many areas they were underperforming, as most Clones weren't built to endure the stress of normal, everyday life). However, as the studies of the human body progressed, the creators of clone armies kept tweaking and improving their "children" in order to boost their effectiveness and make them more suited to the ever-changing face of war. This strife for perfection prompted a rapid advancement in modifying human physiology on a chemical level, which allowed world powers to produce clone soldiers that were practically immune to the common threats of the ABC warfare, such as radiation, gases, and biological agents. Of course, altering their metabolism meant that they were becoming even more susceptible to various other agents, which caused another arms race: to produce and use ABC Clones that were immune to everything the enemy might throw at them, and to discover the enemy vulnerabilities early enough to prepare for such an assault.