Kayla is able to sense physical injury/disease/abnormality in others much as if it were her own. This sense encompasses all things that are 'not normal' for a person, and she can identify their cause(s) and effects with considerable innate accuracy; she has an instinctive and unquestioned familiarity with anatomy, above and beyond the training of a nurse.
Kayla can then 'heal' physical injuries by transferring them from the other person to her own body, leaving the patient hale and whole. The transference itself is near instantaneous; larger/more pervasive injuries take longer to shift, but still only a matter of some seconds, perhaps ten or fifteen at the absolute most. She can take any amount of damage from as many people as necessary within any window of time… up until the point where she dies of the wounds so acquired.
Transference requires skin-to-skin contact; at present, Kayla cannot prevent it from happening once the contact is made. Her awareness of injury is a matter of proximity. Its effective range varies from five feet to one hundred, depending on the severity of injury involved; bruises and minor cuts only at five feet or nearer, injuries of moderate severity at around thirty feet, and mortal wounds she can detect from up to 100 feet away. Painkillers or other medicines do nothing to alleviate what Kayla senses — regardless of which person takes them.
In normal operation, the healing facet of Kayla's power solely encompasses damage at the physiological level: tissues, organs, bones, bacterial infections, and cancerous growths. She cannot affect things that operate at the subcellular level: drugs, toxins, viruses — at least, not directly. (She can detect and diagnose these, however.) Kayla is able to repair damage caused; e.g. a hemotoxic venom destroys cells; Kayla can repair the injured cells but not cleanse the venom. She also cannot repair things that have already fully healed (i.e. scar tissue), are superficial and non-harmful (e.g. tattoos), or are congenital. If her power is amplified sufficiently, however, she may 'heal' virtually anything… except natural aging.
Kayla must then recover from those injuries much as normal people do. However — the power has to have some benefit — she heals at a rate four times faster than the human norm; e.g. a broken bone will be functionally repaired in one week rather than four. This is subject to the same restrictions as her ability to transfer injuries; recovery is accelerated for physical injury, bacterial infection, and such abnormalities as cancer, but viruses, toxins, drugs, and old age affect Kayla just the same as they do any non-Evolved. (Subject to the caveat about side or long-term effects — at the macro level, they still get repaired.) Within the limitations of her accelerated healing ability, Kayla never heals wrong; as long as it's working, she doesn't scar, and she is capable of regenerating. Slowly. Amplification does not increase this rate; it only expands the scope of her power.
Kayla can heal anyone else of a fatal injury if she gets to them in time, but should the wound outstrip her own body's ability to mend it, Kayla dies. She doesn't come back from the dead like certain FCs, either. Dead is dead, except possibly if there's medical intervention. Additionally, all facets of her power apply to animals as well as humans.
