The word viaticum is a Latin word meaning "provisions for a journey," from via, or "way." The Eucharist is seen as the ideal food to strengthen a dying person for the journey from this world to life after death. It seems that originally the Eucharistic bread was placed in the mouth of the dead person so that he or she would have food for what the early Christians believed was a 3 day journey between this world and the next.
The desire to have the consecrated host and eucharistic blood available for the sick and dying led to the reservation of the Blessed Sacrament, a practice which has endured from the earliest days of the Christian Church. Saint Justin Martyr, writing less than fifty years after the death of Saint John the Apostle, mentions that “the deacons communicate each of those present, and carry away to the absent the consecrated Bread, and wine and water.” (Just. M. Apol. I. cap. lxv.)
If the dying person cannot take solid food, the Holy Eucharist may be administered in the form not of bread, but of wine. The sacrament of Anointing of the Sick is often administered immediately before giving Viaticum if a priest is available to do so. Unlike the Anointing of the Sick, Viaticum may be administered by a priest, deacon or extraordinarily (and only in special circumstances) by a lay minister using the reserved Blessed Sacrament.