Jesus said he would be with us always. Catholics believe Christ is present, body, blood, soul, and divinity in the Eucharistic bread.
Corpus Christi began being celebrated in 1263 when in Bolsena, Italy consecrated blood began dripping what appeared to be blood. Modern science believes that this miracle came about through bacteria.
In 750, in Lanciano, Italy a similar miracle occurred during the Eucharist. Science showed that this was indeed human blood (type AB which is common in the Middle East) and that the flesh was also human...cardiac.
The feast of Corpus Christi continues to be celebrated in the Catholic faith.