If you know a Christian with these diseases, see if they have unforgiveness. They cannot be healed if the demons have a right to be there. Cast out unforgiveness and bitterness. Cast out cancer and arthritis. Anoint with oil and pray for healing.
Forgiveness is hard to give because it hurts to extend it to undeserving and hard-hearted ones.
To release a wrong-doer instead of exacting a just penalty requires that we reach out in love, rejecting the temptation to hold bitterness and resentment. We can forget the anger and hurt we felt, but the act is branded in our minds. Anger must be dealt with openly and honestly, not denied or ignored. Either it must be vented in retaliation or the injured party must accept his own anger, bear the burden of it, and confess it in prayer to release himself and to set the other party free. Revenge always hurts the revenger far more than the one at whom it is leveled.