(Portuguese: João de Deus; Spanish: Juan de Dios; born João Duarte Cidade [ˈʒwɐ̃w̃ duˈwaɾ.t siˈða.ðɨ]; March 8, 1495 – March 8, 1550) was a Portuguese soldier turned healthcare worker in Spain, whose followers later formed the Brothers Hospitallers of Saint John of God, a Catholic religious institute dedicated to the care of the poor, sick and those with mental disorders.

Castro began writing in 1579, 29 years after John of God's death, but he did not live to see it published, for he died soon after completing the work.

Shortly after the publication of Castro's Historia, an Italian translation was published at Rome by an Oratorian priest, Giovanni Bordini, in 1587.

When he was about 22 years of age, to escape his master's well-meant but persistent offer of his daughter's hand in marriage, the young man joined a company of foot-soldiers, and in that company he fought for Charles V, Holy Roman Emperor, eventually dispatched by the Count of Oropesa, Fernando Álvarez de Toledo y Zúñiga, against French forces at Fontarabia.

On the way, he befriended a Portuguese knight also traveling there with his wife and daughters, who was being exiled to that region by the King of Portugal for some crime he had committed.

[1] The desertion of one of Cidade's coworkers to a nearby Muslim city in order to escape this treatment (which meant his conversion to that faith) led to a growing feeling of despair in him.

Troubled and feeling spiritually lost from his failure to practice his faith during his years of military service, he went to the Franciscan friary in the colony.

[3] It was during this period of his life that Cidade is said to have had a vision of the Infant Jesus, who bestowed on him the name by which he was later known, John of God, also directing him to go to Granada.

He was incarcerated in the area of the Royal Hospital reserved for the mentally ill and received the treatment of the day, which was to be segregated, chained, flogged, and starved.

[5] Cidade was visited by John of Avila, who advised him to be more actively involved in tending to the needs of others rather than in enduring personal hardships.

[6] Around this time, he made a pilgrimage to the shrine of Our Lady of Guadalupe in Extremadura, where it is said he experienced a vision of Mary, who encouraged him to work with the poor.

[6] When John began to put into effect his dream, because of the stigma attached to mental illness he found himself misunderstood and rejected.

Many stories are related of the heavenly guests who visited him during the early days of his immense tasks, which were lightened at times by the archangel Raphael in person.

The order maintains a presence in 53 countries, operating more than 300 hospitals, services, and centers serving a range of medical needs in addition to mental health and psychiatry.