Viruses are considered non-cellular (or acellular) because they lack a cellular structure. They do not possess the standard components of a cell, such as a nucleus, cytoplasm, organelles, or a plasma membrane. Instead, a virus typically consists of genetic material (DNA or RNA) enclosed within a protective protein coat called a capsid. Because they lack their own metabolic machinery, they are obligate parasites that can only replicate by infecting and using the cellular machinery of a living host.

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