Summary:
- 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.