@Misaka she wouldn't use sacrifice on herself because it wouldn't have the ability to get rid of the sickness it gives her when it's cast and would only make it worse. Meanwhile the MC doesn't have that drawback when he copied her skill so he can heal himself whenever he wants, he just can't use it around people and needs to pretend to have the sickness with his teacher.
WARNING: I may say something horrendously anything-phobic.
Don't take it to heart.
Don't take it to heart.