Short answers, pulled from the story.
The word host derives from the Latin term hostia, which means sacrificial victim. This ancient linguistic root connects the bread used in Christian rituals to the concept of sacrifice found in earlier religious traditions.
Most Eastern Orthodox Churches use leavened bread called prosphora that contains fine white wheat flour, pure water, yeast, and salt. Only a believing Orthodox Christian in good standing may bake the loaves after receiving Confession and engaging in prayer and fasting.
In the Roman Rite, Western Christianity uses thin round unleavened hosts made from wheat flour and water only. The Code of Canon Law requires these hosts be recently made so there is no danger of spoiling.
Catholic theology teaches that at the Words of Institution the bread's substance changes into the Body of Christ. Eastern Christian theology generally views the epiclesis as the point at which the change occurs instead.
Since the 2000s, hosts with low gluten content have been manufactured in the United States, especially in parts of Missouri and New York. Cardinal Joseph Ratzinger wrote a letter in 1995 expanding the Code of Canon Law regarding valid matter to allow low-gluten bread.