You could possibly phrase it something like you know that you have the perseverance, dedication and strength required to succeed in graduate school, because of your previous struggle against the chronic illness, overcoming it, and making certain that it does not define your life, your personality, or your dealings with people. Just be very positive and concentrate on the outcome, while acknowledging that it was a hard time in your life... And hopefully by stressing that, people will be more struck by admiration for you than pity.