Cancer is such a terrible scourge in the world today! But when the believers are called upon to go through such bitter ordeals they have the Faith to sustain them, the love of their Bahá’í friends to comfort them, and the glorious words of Bahá’u’lláh regarding immortality to give them confidence and courage. Blessed are we, indeed, even in the midst of our greatest trials.

Shoghi Effendi, Directives from the Guardian, p. 39

He thanks you for the book you are sending him. He sees no reason why you should not tell the Bahá’ís that cancer seems to be successfully treated by this method sometimes. But as we are a religion and not qualified to pass on scientific matters we cannot sponsor different treatments. We are certainly free to pass on what we have found beneficial to others.

Shoghi Effendi, in The Universal House of Justice, 1997 Aug 13, Science and Religion, p. 3

It is with profoundest sorrow that the Guardian has learned from your letter . . . that your mother is suffering from an acute case of cancer, and he wishes me to hasten to assure you of his brotherly sympathy in this truly grievous situation with which you are faced. He feels that although the physicians have found the disease too advanced, and pronounced her condition as hopeless, you should still endeavour, by means of sustained and concentrated prayer, to bring about her cure . Now that no material efforts can be of any effectiveness you still have one more way open before you, namely that of prayer, and you should therefore continue to supplicate from all your heart that, if it be God’s Will, your mother may be restored to full health, and regain all her forces.

Bahá’u’lláh has revealed special prayers for both spiritual and material healing, and it is therefore your bounden duty to use such prayers on behalf of your mother, and thus put all your trust in the all-sustaining and all-healing power of God.  It matters not whether

you are endowed with high spirituality or not.  The essential thing is that you pray with sincerity and will full purity of heart, and with the one desire of knowing and accepting His Will.  The Guardian too will pray for your mother at the Holy Shrines, that she may be given strength, patience and faith to meet her destiny with absolute courage and complete fortitude.  May the Almighty Giver bestow upon you and your dear mother His richest blessings, sustain you in your activities and cares, strengthen your determination, and fulfil your highest hopes in the service of His Faith.

Shoghi Effendi, Messages to Canada, p. 276

There is no authority for the statement which is alleged to have been said by "‘Abdu’l-Bahá to the effect that cancer is a spiritual disease. The saying to quite unauthoritative, and should not be circulated.

Shoghi Effendi, Lights of Guidance, p. 289