The subject is from the New Testament (John 5). The pool of Bethesda in Jerusalem was periodically visited by an angel, and whoever first stepped into the water after this visit would be cured of illness. Christ visited the pool and a sick man complained to him that he was never able to get to the pool first. When Christ said to him, 'Rise, take up thy bed, and walk', the man was miraculously cured.
The painting was made for the church belonging to the hospital of the Caridad (Charity) in Seville. The Caridad was a charitable brotherhood dedicated to helping the poor and sick of the city. Murillo was himself a member of the brotherhood.
Murillo painted six large pictures for the church representing six of the seven acts of charity (the seventh is represented by a sculpture which remains in place on the high altar). Two of the paintings are still in the church, while the other four, including this one, are now in various museums. This picture represents the act of visiting the sick.