This question triggers many points for review concerning the proper distribution and reception of Holy Communion. First, what should a person do when the Sacred Host– or a piece– by accident falls to the floor? This accident happens even when the Priest, Eucharistic Minister, or recipient is being extremely conscientious. For example, sometimes the Hosts may “stick” together because of humidity or sometimes one just seems to “jump out” of the ciborium. Without question, for someone who is trying to devoutly distribute or receive Holy Communion, seeing the Sacred Host fall on the floor causes a great pain in one’s heart.
In this case, the person– preferably the Priest or Eucharistic Minister– should retrieve it at once, making sure no visible particles are left on the floor. (If there are visible particles, then a linen should be placed over the area where the Host fell, and then the area should be cleaned with water after Mass.) The Priest or Eucharistic Minister may then consume the Sacred Host directly, or isolate it and then after Communion rinse it down the sacrarium (a special sink in the sacristy which empties directly into the ground not the sewer system). Oftentimes, the recipient who dropped the Sacred Host picks it up and consumes it immediately; here again the Priest, Eucharistic Minister, and recipient should be conscious of any remaining particles. (See General Instruction of the Roman Missal, #280.)
Second, the scenario presented in the question also prompts a good review. (1) Priests and Eucharistic Ministers must be extremely alert during the distribution of Holy Communion, making sure the recipients consume the whole Sacred Host and immediately retrieving it if an accident occurs.
(2) When receiving Holy Communion, a recipient must put forth his tongue enough so that the Sacred Host may be placed securely on it, or make a proper throne for the Lord with the hands so that it may rest securely. If receiving on the hand, the recipient should side-step, still facing the altar, and consume the Sacred Host before turning to go back to the pew. Most communion accidents occur because the person begins walking back toward the pew, looking ahead, rather than focusing on receiving our Lord. Better to pause to receive, adore, and consume the Sacred Host, than to rush like in a cafeteria line and risk dropping it. No one should be facing the pews or walking back to the pews when receiving Holy Communion. Unfortunately, in many parishes, the distribution of Holy Communion has been turned into a fast-paced assembly line rather than allowing a person to commune with the Lord received in this precious Sacrament.
Lastly, if a person did notice a Sacred Host, or even a piece, fall to the floor, he should retrieve it and give it to the Priest at an appropriate time– maybe at the end of Communion when the Priest prepares to purify the vessels.
Accidents surrounding the distribution and reception of Holy Communion do occur. We all must be very conscious of our actions. Never must we become lax in our approach to Holy Communion; rather, we must always be reverent and even protective, for we have the great privilege of receiving the Body, Blood, Soul and Divinity of our Savior truly present in the Holy Eucharist.