There are two ways of healing sickness, material means and spiritual means. The first is by the treatment of physicians; the second consisteth in prayers offered by the spiritual ones to God and in turning to Him. Both means should be used and practised.
Illnesses which occur by reason of physical causes should be treated by doctors with medical remedies; those which are due to spiritual causes disappear through spiritual means. Thus an illness caused by affliction, fear, nervous impressions, will be healed more effectively by spiritual rather than by physical treatment. Hence, both kinds of treatment should be followed; they are not contradictory. Therefore thou shouldst also accept physical remedies inasmuch as these too have come from the mercy and favour of God, Who hath revealed and made manifest medical science so that His servants may profit from this kind of treatment also. Thou shouldst give equal attention to spiritual treatments, for they produce marvellous effects.
Now, if thou wishest to know the true remedy which will heal man from all sickness and will give him the health of the divine kingdom, know that it is the precepts and teachings of God. Focus thine attention upon them. (‘Abdu'l-Baha, Selections from the Writings of ‘Abdu'l-Baha)