Virology 101

What are viruses?

Viruses are acellular pathogens, meaning they are not cells and they are not made up of cells, and they have the ability to cause disease. Minimally, every virus has nucleic acid and a protein coat.

Viruses are unique for the following reasons:

  1. No Metabolism, viruses have no ability to make energy on their own
  2. Some viruses can use RNA as their genomic material
  3. Viruses are made from small repetitive subunits that cannot grow in size (no organelles)
  4. Viruses have no ability to make their own proteins (all bacteria do)

Historical Background:

The 1800’s were the golden Era of Bacteriology/Microbiology due to Louis Pasteur’s swan neck flask experiment that disproved spontaneous generation. Moreover, all research and experiments in the scientific world, up to the1930’s were based on preconceived notions that everything that caused disease was a bacterium. For example, back then scientists tried to make all disease-causing microbes fit “Koch’s Postulates.”

Koch’s Postulates:

  1. Disease causing organism should be found in every case there is disease
  2. *Should be able to isolate it and grow it in pure culture
  3. Should be able to put it back in new host and cause disease
  4. Should be able to re-isolate pathogen

Unknown to the scientists of the time viruses do not work that way. Sometimes the virus cannot be found because it is latent (which we will discuss later). Sometimes the virus cannot be isolated or grown in pure culture.

It wasn’t until the 1890’s that the first virus was discovered, the Tobacco Mosaic Virus (TMV). The discovery of this virus was driven by economical concerns. Tobacco was a staple crop and if the tobacco was being diminished by TMV, it was bad for business. Furthermore the virus was costing the government a lot of money so the government funded the research that led to this discovery.

How the first virus was discovered:

The research involved grinding up leaves and making liquid extract. The liquid extract could cause disease in another tobacco plant. The liquid extract caused the disease to be contagious. Additionally, a scientist conducted a filtration experiment in which he would continually filter the liquid extract to see if it still caused disease. After filtering the liquid, he found that the filtered extract was still disease-causing. This was the first sign that the disease was not bacterial. Although they were not aware of very small bacteria back then, in general bacteria are filterable. Critics stated that the tobacco plant’s disease was caused by a bacterial toxin. However, if it were a toxin, after the toxin had been diluted out to a certain point it could no longer cause disease. However a virus can re-concentrate disease power. Contrarily, toxins cannot be spontaneously created but a virus can proliferate. This is known as the “Re-concentration experiment.”

The first animal virus discovered:

First animal virus discovered was the “Foot and mouth” virus. This virus was discovered the same way that TMV was discovered. Scientists found that they could re-amplify diluted stock in cattle, it could not be grown in culture, and it was filterable.

The first human virus discovered:

The first human virus discovered is the Yellow Fever virus. The discovery of this virus, like the discovery of TMV was influenced by  economic concerns. This virus was a major problem for the United States because it interfered with the country’s ability to build the Panama Canal. When scientists were able to find a cure to yellow fever, America became an even bigger international power and isolationism ended.

The discovery of viruses was one of the most important progressions made in scientific history. When the electron microscope was created in the early 1930’s the virus was viewable (because viruses are so small, you would not be able to see them with a  compound microscope). Because the electron microscope allowed scientists to see what a virus actually looked like, viruses became more than a filterable aliquot of a contagion. Now, scientists realize that there is something other than bacteria out there.


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