Introduction to Virology

Course: Virology

Lecturer: Dr. Weam Saad
Lecture: Introduction to Virology

Historical Overview:

The word virus is from the Latin word vīrus referring to poison. The scientist Louis Pasteur was unable to find the causative agent for rabies and said it was a pathogen too small to be detected using a microscope.
The scientist Charles Chamberland invented a filter (known today as the HYPERLINK "https://en.wikipedia.org/wiki/Chamberland_filter" \o "Chamberland filter" Chamberland filter or the Pasteur-Chamberland filter) with pores smaller than bacteria. The solutions containing bacteria pass through the filter and completely remove bacteria. In 1892, the Russian biologist Dmitri Ivanovsky used this filter to study what is now known as the tobacco mosaic virus. His experiments showed that crushed leaf extracts from infected tobacco plants remain infectious after filtration. Ivanovsky suggested the infection might be caused by a toxin produced by bacteria and this was part of the germ theory of disease. The microbiologist HYPERLINK "https://en.wikipedia.org/wiki/Martinus_Beijerinck" \o "" Martinus Beijerinck observed that the agent multiplied only in cells that were dividing and called it soluble living germ, and re-introduced the word virus.
The first image of a virus was obtained after the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll for the virus tobacco mosaic virus.

Tobacco mosaic virus

Viruses are found wherever there is living cells. The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses. In addition, viral genetic material may occasionally integrate into the germline of the host cell, and they can be passed on vertically to the offspring of the host for many generations. This provides a source of information to trace back ancient viruses that have existed up to millions of years ago.

Origin of Viruses:

There are three hypotheses that explain the origins of viruses:
Regressive hypothesis: Viruses may be small cells that parasitized larger cells and over time, genes not required were lost. This hypothesis is supported by the intracellular parasite bacteria Rickettsia sp. and Chlamydia sp. Because they are living cells that like viruses, reproduce only inside host cells.
Cellular origin hypothesis
Some viruses may be evolved from bits of DNA or RNA that escaped from the genes of a larger host cells like plasmids (pieces of naked DNA that can move between cells) or transposons (molecules of DNA that replicate and move around to different positions within the genes of the cell). The jumping genes, transposons are examples of mobile genetic elements and could be the origin of some viruses.
Co-evolution hypothesis
It is the virus-first hypothesis that viruses are evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on Earth and shared in cellular life for billions of years.


As the Earth warms due to climate change during 18th and 19th Centuries; many permafrost soils are melting after thousands years of frozen, and as the soils melt they are releasing ancient viruses and bacteria that were preserved for long time, these pathogens caused global epidemics in the past and can infect humans or animals and the deadly outbreaks may come back.