Viruses vs. Living Organisms: Two Fundamental Differences

Viruses differ from cellular life in two essential ways:

1. Non-cellular structure: Viruses lack cells, organelles, cytoplasm, and metabolism. They are nucleic acid (DNA or RNA) packaged in a protein capsid (± envelope). Size: 20–300 nm vs. cells (10–100 µm).
2. Obligate intracellular parasitism: Viruses cannot replicate independently. They must hijack host cell machinery (ribosomes, polymerases, ATP, nucleotides) to copy their genome and assemble progeny. No energy generation, no protein synthesis, no binary fission.

These features place viruses at the edge of life—genetic elements that evolve but require living cells to propagate.