Epiphany

Although she was particularly bright and energetic, the only thing that set Sally Marston apart from the other children her age was that she never seemed to get sick. Although neither she nor her family realized it, Sally’s immune system was more adaptable than a normal person’s, targeting illnesses before she suffered any symptoms. This quirk of genetics would save her life the day SHADOW attacked her Midwestern hometown.
A test of an experimental bio-weapon escalated as AEGIS arrived, and in the ensuing battle a wash of toxic microbes and acidic gas was deployed over her high school. Sally’s entire class was rushed to nearby hospitals with severe injuries. Few of the doused survived, and most of the survivors had permanent injuries as their organs failed and their limbs atrophied. Sally alone survived without injury, her body altering itself to keep her alive against these new dangers. Unknown to Sally at the time, the exposure had so severely damaged her body that her mutant immune system had reinvented itself to adapt. Now able to rebuild structures from almost nothing, it hardened her flesh and bone, and packed dense musculature into her small frame. After an early adventure nearly drowned her, her body even developed gills to compensate.
Not terribly adventurous, Sally enrolled in medical school, hoping to someday help her classmates recover from their terrible injuries. There she discovered her body could adapt itself to any new stress, including rigorous study, and within a few years she had developed from a bright but ordinary child into one of the foremost minds on the planet. She originally joined UNIQUE as a reserve member to apply her specialized knowledge of superhuman physiology and medicine, but her skills—both mental and physical—proved invaluable enough that she recently accepted a position as an active member.