Catholics believe that the bread and wine at Mass are transformed into Jesus' body, blood, soul and divinity. It is a mystery known as "transubstantiation," which means that while the appearances of bread and wine remain, the underlying substance is changed (through the power of God) completely to the body and blood of Christ. It is a teaching based on scripture and tradition and has remained unchanged in its essence since apostolic times.
However, the Church recognizes that on rare occasions both the substance and the appearance is changed into Jesus' body and blood. These are identified as "Eucharistic miracles," and testify to the words Jesus gave to his disciples at the Last Supper ("This is my body").
One such miracle occurred in Santarem, Portugal, in 1247, when a young woman, jealous of her husband, went to a sorceress in hopes of making a love potion. The sorceress instructed the young woman to obtain a consecrated host from a Catholic Church. The young woman followed the instructions and hid the host in a veil that covered her head.
Shortly after she enclosed the host in her veil, it began to bleed. She was frightened by what she saw and quickly shut the host in a wooden trunk in her bedroom. At night, brilliant rays of light came from the trunk and she was forced to tell her husband everything.
The next day she brought the miraculous host to the parish priest, who enshrined the bleeding host in a reliquary. Canonical investigations were undertaken in 1340 and 1612 and on both occasions the miracle was found to be authentic.
Over the centuries the host has appeared like fresh bloody tissue, or dry up and harden. Solidified Blood from the miracle is kept in a glass vial in a reliquary at the Shrine in Santarem above the main altar.
It is a sign for all of Jesus' real presence in the Eucharistic host and has reinvigorated the faith of many Catholics around the world.
