Viruses are particles composed of an internal core containing either DNA or RNA (but not both) covered by a protective protein coat.

Some viruses have an outer lipoprotein membrane, called an envelope, external to the coat.

Viruses do not have a nucleus, cytoplasm, mitochondria, or ribosomes.

Viruses must reproduce (replicate) within cells, because they cannot generate energy or synthesize proteins. Because they can reproduce only within cells, viruses are obligate intracellular parasites.