Equality is a very important part of this yoga; it is necessary to keep equality under pain and suffering — and that means to endure firmly and calmly, not to be restless or troubled or depressed or despondent, to go on with a steady faith in the Divine Will.
But equality does not include inert acceptance. If, for instance, there is temporary failure of some endeavour in the sadhana, one has to keep equality, not to be troubled or despondent, but one has not to accept the failure as an indication of the Divine Will and give up the endeavour. You ought rather to find out the reason and meaning of the failure and go forward in faith towards victory.
So with illness — you have not to be troubled, shaken or restless, but you have not to accept illness as the Divine Will, but rather look upon it as an imperfection of the body to be got rid of as you try to get rid of vital imperfections or mental errors.
Ref: Letters on Yoga – II