More information available from the National Shrine of St. Rita of Cascia.
At Cascia, in the basilica dedicated to St. Rita, is also preserved the relic of the Eucharistic Miracle, which happened near Siena in 1330. A priest was asked to bring Communion to a sick peasant. The priest took a consecrated Host which he irreverently placed in the pages of his breviary and went to the peasant. When he arrived at the house of the sick man, after hearing his confession, he opened the book to take out the Host which he had placed there. To his great surprise hefound that the Host was stained with living blood, so much as to mark both pages between which the Blessed Sacrament had been placed. The priest, confused and penitent, went immediately to Siena to the Augustinian Priory to ask the counsel of Fr. Simone Fidati of Cascia, known by all to be a holy man. Fr. Fidati, having heard the story, granted pardon to the priest and asked to keep the two pages marked by Blood. Many popes have promoted veneration, conceding indulgences.
~ Excerpt and more information, including pictures, from here. ~