Any of the elementary agents that possess some of the properties of living systems, such as having a genome and being able to adapt to changing environments. However, viruses are not functionally active outside their host cells. Viruses share three characteristics: (1) their simple, acellular organization consisting of a nucleic acid genome surrounded by a protective protein shell, which may itself be enclosed within an envelope that includes a membrane; (2) the presence of either DNA or RNA, but not both; and (3) their inability to reproduce independent of host cells. In essence, viruses are nucleic acid molecules, that is, genomes that can enter cells, replicate in them, and encode proteins capable of forming protective shells around them.