The doctrine of concomitance states that the risen Savior is wholly present under both species (consecrated bread and consecrated wine).
Catholics receive the Body, Blood, Soul, and Divinity of the glorified Lord in Holy Communion. The priest separately consecrates the wafers of (wheat) bread and then separately consecrates the chalice of (grape) wine to symbolize the separation of body and blood, so he can sacramentally reenact the sacrifice of Calvary where Jesus shed his blood and was crucified in his body. Yet, it is not dead flesh and blood which is received in Holy Communion, but the risen body and blood of Christ. What was separated in death is united in the resurrection.
Catholic dogma is that both the body and blood of Christ are contained in each one—the Sacred Host (consecrated bread) and the Precious Blood (consecrated wine). Hence, if someone only receives the Sacred Host, they are in reality getting both the body and the blood.
Otherwise, if it was still separated, He would still be dead. Christ is risen, however, so wherever His body is there is also His blood, and vice versa. Martin Luther and other Protestant Reformers taught that the faithful had to receive both species (forms) of Holy Communion. The Council of Trent in the sixteenth century defined that only the priest had to consume both to complete the sacrifice of the sacrament. The faithful could just receive the Sacred Host and not the Chalice of Precious Blood since in either one is both of them (body and blood of Christ).
People with celiac disease or severe allergies to wheat or gluten can be given only the Precious Blood (consecrated wine), just as alcoholics can take only the Sacred Host. Neither one would be getting half of Jesus. Each one would get the fullness of the Body and Blood, Soul and Divinity of Christ. The appearances of bread and wine remain, but the substances have been miraculously changed by God (through the priest saying the exact words of Christ at the Last Supper, “This is my body” and “This is my blood”).
Since the Second Vatican Council (1962–1965) and more recently since the new guidelines in the Roman Missal of 2000, the Church highly encourages Catholics to partake of both sacred species as a more complete sign value (referring to the actions of eating and drinking); however, care must be taken that no one fall into the error of thinking that they must receive both the Sacred Host and the Chalice of Precious Blood in order to fully receive the body and blood of Christ. Practical, pastoral, or medical concerns may prevent Holy Communion under the species of consecrated wine, in which case Rome has said that any priest is allowed to give only the host and not offer both host and chalice to all the communicants since in either element, the fullness of Christ resides.
Number 85 of the General Instruction of the Roman Missal reads, “It is most desirable that the faithful, just as the priest himself is bound to do, receive the Lord’s body from host consecrated at the same Mass, and that in the instances when it is permitted, they partake of the chalice so that even by means of the signs, Communion will stand out more clearly as a participation in the sacrifice actually being celebrated.”
If there are not enough Extraordinary Ministers of Holy Communion to help distribute, however, especially when there is a large crowd, then the priest and deacon (who are the ordinary ministers of Holy Communion) must only give the host. Even though colds and the flu may not be spread through the common cup, it may be wise to dispense with the chalice during such times of illness. The reason for this precaution is that no matter what medical evidence you have to the contrary, most people still think otherwise and will just avoid drinking the Precious Blood during flu season. The alcohol content alone should take care of any potential contagion, but some recent strains of more serious and possibly epidemic level infections have made a lot of people nervous about drinking from the same cup.
Ironically, physicians say more germs are transmitted and caught through the hands and handshaking. Rather than having too much leftover Precious Blood (due to a very few communicants receiving from the chalice) at the end of Mass, some priests and parishes just offer it occasionally, once a month or on special feast days, rather than at every Mass. Any leftover consecrated wine must be consumed immediately and cannot be kept in the tabernacle with the consecrated hosts, nor can it be poured down the special drain in the sacrisity (called the sacrarium).
Discretionary practices should be employed in such times by the main celebrant. In Saint Peter’s in Rome and many of the Pontifical Celebrations, due to large numbers, only the host is distributed.