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Discovery and Origin of Viruses

Introduces how viruses were first identified.

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Discovery and Origin of Viruses

Can you "discover" something without actually seeing it?

Well, yes you can, and that's precisely how viruses were discovered. Viruses are much smaller than bacteria, so special microscopes are needed to see them, but the existence of viruses was known prior to the development of these special microscopes.

Discovery and Origin of Viruses

Viruses are so small that they can be seen only with an electron microscope. Before electron microscopes were invented, scientists knew viruses must exist. How did they know? They had demonstrated that particles smaller than bacteria cause disease.

Discovery of Viruses

Researchers used special filters to remove bacteria from tissues that were infected. If bacteria were causing the infection, the filtered tissues should no longer be able to make other organisms sick. However, the filtered tissues remained infective. This meant that something even smaller than bacteria was causing the infection.

Scientists did not actually see viruses for the first time until the 1930s. That’s when the electron microscope was invented. In 1915, English bacteriologist Frederick Twort discovered bacteriophage, the viruses that attack bacteria. He noticed tiny clear spots within bacterial colonies, and hypothesized that something was killing the bacteria. The tobacco mosaic virus shown in Figure below was the first one to be seen.

Tobacco Mosaic Virus under a microscope

Tobacco Mosaic Virus. This tobacco mosaic virus was the first virus to be discovered. It was first seen with an electron microscope in 1935.

Origin of Viruses

Where did viruses come from? How did the first viruses arise? The answers to these questions are not known for certain. Several hypotheses have been proposed. The two main hypotheses are stated below. Both may be valid and explain the origin of different viruses.

  • Small viruses started as runaway pieces of nucleic acid that originally came from living cells such as bacteria.
  • Large viruses were once parasitic cells inside bigger host cells. Over time, genes needed to survive and reproduce outside host cells were lost.


  • Viruses were assumed to exist before they were first seen with an electron microscope in the 1930s.
  • Multiple hypotheses for viral origins have been proposed.


  1. Why did scientists think viruses must exist even before they ever saw them with an electron microscope?
  2. State two hypotheses for the origin of viruses.
  3. What allowed scientists to see a virus for the first time?




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Virus that attack bacteria.

electron microscope

A type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image.

tobacco mosaic virus

Single stranded RNA virus that infects plants, especially tobacco; the first virus to be discovered.

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