• Viruses were assumed to exist before they were first seen with an electron microscope in the 1930s.
  • English bacteriologist Frederick Twort discovered bacteriophage, the viruses that attack bacteria.
  • The tobacco mosaic virus shown in Figure below was the first one to be seen.
  • Several hypotheses for viral origins have been proposed.
  • Both may be valid and explain the origin of different viruses.
  • The two main hypotheses are stated below and explain how viruses were discovered.
  • The origin of viruses is still not known for certain.
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