Viruses: Definition, Origin, & Milestones

Definition of Virus

• Non-cellular, infectious entities.
• Contain either RNA or DNA, encased in a protein coat (capsid).
• Replicate only inside living cells.

Historical Evidence & Characteristics

• Term originally linked to infectious diseases of unknown cause; derived from Latin “venom” (poison).
• First clue: C. Chamberland observed rabies agent passed through porcelain filters (pore size 100!\text{–}!1000\ \text{nm}).

Origin Theories (Critical Concept)

• Regressive hypothesis: Viruses evolved from free-living cells / intracellular parasites.
• Progressive hypothesis: Arose from self-replicating RNA or DNA (mobile genetic elements) that escaped host cells.
• Virus-first hypothesis: Viruses pre-dated the first cellular life as earliest self-replicating entities.

Key Contributions to Virology

• E. Jenner (1796): First vaccine (smallpox).
• C. Chamberland (1884): Demonstrated filterable nature of rabies virus.
• D. Ivanowski (1892): Identified filterable agent causing Tobacco Mosaic Disease.
• Twort & D’Herelle (1915,\ 1917): Discovered bacteriophages.
• W. M. Stanley (1935): Isolated, purified, and crystallized Tobacco Mosaic Virus (TMV).