Viruses are classified along four independent criteria — size, capsid shape, the single type of nucleic acid they carry, and host range — because they are acellular obligate intracellular parasites that lack organelles and therefore cannot generate ATP or replicate autonomously. Formally, a virus consists of a capsid built from identical protein subunits called capsomers enclosing a genome of exactly one nucleic acid type (double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA), the assembly of capsid plus genome being termed a nucleocapsid, which may optionally be surrounded by a lipid envelope. This taxonomy belongs to microbiology and virology, and it links viral structural morphology to the entry mechanisms — injection through a tail sheath, receptor-mediated endocytosis, or direct membrane fusion — by which viruses exploit host cellular machinery.
Viruses are classified along four independent criteria — size, capsid shape, the single type of nucleic acid they carry, and host range — because they are acellular obligate intracellular parasites t…