Virus is a microscopic organism that lives in a cell of another living thing. Although viruses are extremely small and simple, they are a major cause of disease. Some viruses infect human beings with such diseases as measles, influenza, and the common cold.
Passive Mobs. Passive mobs are composed of creatures that will never, under any circumstances, attack the player. Some passive mobs can be bred to create more of them, e.g. pigs can be bred to effectively farm them.
The grinder kills the mobs outright or, through fall damage, harms them enough that they can be killed with a single blow, thus quickly accruing you experience points. Large-scale mob farms can spawn hundreds of mobs per minute, grind them up like so much raw meat, and deliver their drops through an automated collection system into a bundle of chests.