A - Thanks for the question.
It is not necessary for us to take both species of the Eucharist (a species would be either the Body of Christ or the Precious Blood), because we receive the full presence of Christ with either one or the other. The Catechism says:
"Christ is present whole and entire in each of the species and whole and entire in each of their parts, in such a way that the breaking of the bread does not divide Christ" - CCC 1377Thus, it is a prudential decision by the Bishop of each diocese to ask parishes not to offer the communal chalices to the congregation in order to prevent the spread of communicable diseases. In 2009, the swine flu caused the Austin Diocese to issue a statement which read:
April 28, 2009I hope this helps explain this issue.
Priests and Deacons of the Diocese of Austin
Dear Brothers in Christ:
I have been in conversation with members of the State of Texas Health Department. There seems to be more unknown, than actually known about the swine flu virus. We do not want to create a “panic” but the reality remains that the virus has infected people in Texas and many unanswered questions remain. Therefore, it is wise to be cautious. For the sake of caution and respect for life, I am requesting that we do not offer communion under both species until further notice. Please do not offer the Blood of Christ at Eucharist until more is known about the virus. It seems that having the public drink from the chalice may be an unnecessary risk. This is also an opportunity to make sure all ministers of communion, ordinary and extraordinary, should have clean hands. Thanks for your cooperation. Let us pray for those whose lives have been taken in death and for those fighting the virus.
Fraternally in Christ,
Most Reverend Gregory M. Aymond
Bishop of Austin
Also, it is not necessary to offer the Precious Blood, though it is recommended. The early Church offered Communion under both kinds, but starting in the 11th century and up until Vatican II, the norm and custom was to only offer the hosts. This is what was written in Vatican II about Communion under both kinds:
"The dogmatic principles which were laid down by the Council of Trent remaining intact, Communion under both kinds may be granted when the bishops think fit, not only to clerics and religious, but also to the laity, in cases to be determined by the Apostolic See." - SC 55Later, the General Instruction of the Roman Missal makes it even more permissive:
"Holy Communion has a more complete form as a sign when it is received under both kinds. For in this manner of reception a fuller sign of the Eucharistic banquet shines forth. Moreover there is a clearer expression of that will by which the new and everlasting covenant is ratified in the blood of the Lord and of the relationship of the Eucharistic banquet to the eschatological banquet in the Father's kingdom." -GIRM 281Since the GIRM's instructions, the US Bishops have issued guidelines about distribution under both species.