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.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, p. 152)

If the health and well-being of the body be expended in the path of the Kingdom, this is very acceptable and praiseworthy; and if it is expended to the benefit of the human world in general – even though it be to their material benefit and be a means of doing good – that is also acceptable. But if the health and welfare of man be spent in sensual desires, in a life on the animal plane, and in devilish pursuits – then disease is better than such health; nay, death itself is preferable to such a life. If thou art desirous of health, wish thou health for serving the Kingdom.
(`Abdu’l-Baha: Baha’i World Faith, p. 376)

Material and Spiritual Healing:
All true healing comes from God! There are two causes for sickness, one is material, the other spiritual. If the sickness is of the body, a material remedy is needed, if of the soul, a spiritual remedy.
If the heavenly benediction be upon us while we are being healed then only can we be made whole, for medicine is but the outward and visible means through which we obtain the heavenly healing. Unless the spirit be healed, the cure of the body is worth nothing. All is in the hands of God, and without Him there can be no health in us!
(`Abdu’l-Baha: Paris Talks, p. 19)

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.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, pp. 151-152)

Matters related to man’s spirit have a great effect on his bodily condition. For instance, thou shouldst impart gladness to thy patient, give him comfort and joy, and bring him to ecstasy and exultation. How often hath it occurred that this hath caused early recovery. Therefore, treat thou the sick with both powers. Spiritual feelings have a surprising effect on healing nervous ailments.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, pp. 150-151)

Know that there are four kinds of curing and healing without medicine. Two are due to material causes, and two to spiritual causes.
Of the two kinds of material healing, one is due to the fact that in man both health and sickness are contagious. The contagion of disease is violent and rapid, while that of health is extremely weak and slow. If two bodies are brought into contact with each other, it is certain that microbic particles will pass from one to the other. In the same way that disease is transferred from one body to another with rapid and strong contagion, it may be that the strong health of a healthy man will alleviate a very slight malady in a sick person. That is to say, the contagion of disease is violent and has a rapid effect, while that of health is very slow and has a small effect, and it is only in very slight diseases that it has even this small effect. The strong power of a healthy body can overcome a slight weakness of a sick body, and health results. This is one kind of healing.
The other kind of healing without medicine is through the magnetic force which acts from one body on another and becomes the cause of cure. This force also has only a slight effect. Sometimes one can benefit a sick person by placing one’s hand upon his head or upon his heart. Why? Because of the effect of the magnetism, and of the mental impression made upon the sick person, which causes the disease to vanish. But this effect is also very slight and weak.
Of the two other kinds of healing which are spiritual – that is to say, where the means of cure is a spiritual power – one results from the entire concentration of the mind of a strong person upon a sick person, when the latter expects with all his concentrated faith that a cure will be effected from the spiritual power of the strong person, to such an extent that there will be a cordial connection between the strong person and the invalid. The strong person makes every effort to cure the sick patient, and the sick patient is then sure of receiving a cure. From the effect of these mental impressions an excitement of the nerves is produced, and this impression and this excitement of the nerves will become the cause of the recovery of the sick person. So when a sick person has a strong desire and intense hope for something and hears suddenly the tidings of its realization, a nervous excitement is produced which will make the malady entirely disappear. In the same way, if a cause of terror suddenly occurs, perhaps an excitement may be produced in the nerves of a strong person which will immediately cause a malady. The cause of the sickness will be no material thing, for that person has not eaten anything, and nothing harmful has touched him; the excitement of the nerves is then the only cause of the illness. In the same way the sudden realization of a chief desire will give such joy that the nerves will be excited by it, and this excitement may produce health.
To conclude, the complete and perfect connection between the spiritual doctor and the sick person – that is, a connection of such a kind that the spiritual doctor entirely concentrates himself, and all the attention of the sick person is given to the spiritual doctor from whom he expects to realize health – causes an excitement of the nerves, and health is produced. But all this has effect only to a certain extent, and that not always. For if someone is afflicted with a very violent disease, or is wounded, these means will not remove the disease nor close and heal the wound – that is to say, these means have no power in severe maladies, unless the constitution helps, because a strong constitution often overcomes disease. This is the third kind of healing.
But the fourth kind of healing is produced through the power of the Holy Spirit. This does not depend on contact, nor on sight, nor upon presence; it is not dependent upon any condition. Whether the disease be light or severe, whether there be a contact of bodies or not, whether a personal connection be established between the sick person and the healer or not, this healing takes place through the power of the Holy Spirit.
(`Abdu’l-Baha: Some Answered Questions, pp. 254-256)

The healing that is by the power of the Holy Spirit needs no special concentration or contact. It is through the wish or desire and the prayer of the holy person. The one who is sick may be in the East and the healer in the West, and they may not have been acquainted with each other, but as soon as that holy person turns his heart to God and begins to pray, the sick one is healed. This is a gift belonging to the Holy Manifestations and those who are in the highest station.
(`Abdu’l-Baha: Health and Healing, p. 475)

The prayers which were revealed to ask for healing apply both to physical and spiritual healing. Recite them, then, to heal both the soul and the body. If healing is right for the patient, it will certainly be granted; but for some ailing persons, healing would only be the cause of other ills, and therefore wisdom doth not permit an affirmative answer to the prayer.
O handmaid of God! The power of the Holy Spirit healeth both physical and spiritual ailments.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, pp. 161-162)

Treatment by Physicians:
Resort ye, in times of sickness, to competent physicians; We have not set aside the use of material means, rather have We confirmed it through this Pen, which God hath made to be the Dawning-place of His shining and glorious Cause.
(Baha’u’llah: The Kitab-i-Aqdas, p. 60)

Whatever competent physicians or surgeons prescribe for a patient should be accepted and complied with, provided that they are adorned with the ornament of justice. If they were to be endued with divine understanding, that would certainly be preferable and more desirable.
(Baha’u’llah: Health and Healing, p. 459)

Do not neglect medical treatment when it is necessary, but leave it off when health has been restored…. Treat disease through diet, by preference, refraining from the use of drugs; and if you find what is required in a single herb, do not resort to a compounded medicament. Abstain from drugs when the health is good, but administer them when necessary.
(Baha’u’llah: Health and Healing, p. 460)

…Baha’u’llah says to consult the best physicians, in other words, doctors who have studied a scientific system of medicine;…
(Shoghi Effendi: Directives of the Guardian, p. 36)

Healing in the Future:
Now let us speak of material healing. The science of medicine is still in a condition of infancy; it has not reached maturity. But when it has reached this point, cures will be performed by things which are not repulsive to the smell and taste of man – that is to say, by aliments, fruits and vegetables which are agreeable to the taste and have an agreeable smell. For the provoking cause of disease – that is to say, the cause of the entrance of disease into the human body – is either a physical one or is the effect of excitement of the nerves.
But the principal causes of disease are physical, for the human body is composed of numerous elements, but in the measure of an especial equilibrium. As long as this equilibrium is maintained, man is preserved from disease; but if this essential balance, which is the pivot of the constitution, is disturbed, the constitution is disordered, and disease will supervene.
(`Abdu’l-Baha: Some Answered Questions, p. 257)

When highly-skilled physicians shall fully examine this matter, thoroughly and perseveringly, it will be clearly seen that the incursion of disease is due to a disturbance in the relative amounts of the body’s component substances, and that treatment consisteth in adjusting these relative amounts, and that this can be apprehended and made possible by means of foods.
It is certain that in this wonderful new age the development of medical science will lead to the doctors’ healing their patients with foods.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, pp. 154-155)

At whatever time highly-skilled physicians shall have developed the healing of illnesses by means of foods, and shall make provision for simple foods, and shall prohibit humankind from living as slaves to their lustful appetites, it is certain that the incidence of chronic and diversified illnesses will abate, and the general health of all mankind will be much improved.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, p. 156)

Soul, Mind, and Body:
Know thou that the soul of man is exalted above, and is independent of all infirmities of body or mind. That a sick person showeth signs of weakness is due to the hindrances that interpose themselves between his soul and his body, for the soul itself remaineth unaffected by any bodily ailments. Consider the light of the lamp. Though an external object may interfere with its radiance, the light itself continueth to shine with undiminished power. In like manner, every malady afflicting the body of man is an impediment that preventeth the soul from manifesting its inherent might and power.
(Baha’u’llah: Gleanings, pp. 153-154)

For the mind to manifest itself, the human body must be whole; and a sound mind cannot be but in a sound body, whereas the soul dependeth not upon the body.
(`Abdu’l-Baha: Tablet to August Forel, p.8)

…you should not neglect your health, but consider it the means which enables you to serve. It – the body – is like a horse which carries the personality and spirit, and as such should be well cared for so it can do its work! You should certainly safeguard your nerves, and force yourself to take time, and not only for prayer and meditation, but for real rest and relaxation….
(Shoghi Effendi: Health, Healing, …., p. 482)