Virology: Introduction and History

Virology is the study of viruses.
Viruses are either human, animal, bird, bacterial, insect, or mycoplasma.
Human viruses include Small pox, HIV(AIDS), Hepatitis A, B, C, D, E, G, Poliomyelitis, and Rabies.
Animal viruses include RVF, FMD, RP, BVD.
Bird viruses include NDV, Gumboro, IBV, ILT.
Bacterial viruses are bacteriophages.
Insect viruses are baculovirus.
Mycoplasma are mycoplasmalphages.

Viruses are very small obligatory intracellular parasites.
They lack metabolic enzymes and depend on host cells for replication.
Viruses are 20-300nm in size (nm = 10⁻⁷ cm).
They are devoid of machinery for energy production and macromolecule biosynthesis.

Ancient civilizations were aware of virus effects and conducted research into causes and prevention.
The oldest documented record of a virus infection dates back to approximately 3700 BCE, depicting a temple priest with paralytic poliomyelitis symptoms discovered in Memphis, Egypt.
Evidence suggests Pharaoh Ramses V (1196 BCE) likely had smallpox.
Smallpox was endemic in China by 1000 BCE, leading to the development of variolation practices involving inhalation or inoculation of pus from lesions into scratches.

In the late 19th century, a diverse microbial world of bacteria, fungi, and protozoa was recognized.
Jacob Henle hypothesized the existence of infectious agents too small to be seen under a light microscope, capable of causing diseases.
This concept wasn't widely accepted at the time due to lack of evidence

Adolf Mayer identified a disease (tobacco mosaic disease) affecting tobacco plants and linked it to a causative agent.
Through experiments, he demonstrated that extracts from diseased plants could transmit the disease to healthy ones.

Louis Pasteur developed methods for producing attenuated rabies virus preparations.
This method involved progressively drying spinal cords of infected rabbits and inoculating them into animals to provide protection against a virulent form of the virus.
The resulting vaccine was the first artificially produced one and further research lead Koch who came up with postulates to identify the agent responsible for the infection.

Dmitri Iwanowski discovered in 1892 that extracts from diseased plants could transmit a disease even after being filtered to remove known bacteria.
Martinus Beijerinick confirmed Iwanowski's findings, and is credited with developing the modern concept of a virus.
Friedrich Loeffler and Paul Frosch linked a similar agent to foot-and-mouth disease. This demonstrated that these agents affected both plants and animals.
Landsteiner and Popper showed poliomyelitis was caused by a filterable agent, making it the first human disease identified as having a viral cause.

Frederick Twort and Felix d'Herelle discovered bacteriophages (viruses that infect bacteria).
These bacteriophages became valuable model systems for exploring various aspects of virology, including viral structure, genetics, and replication.
Walter Reed demonstrated that yellow fever was caused by a virus spread by mosquitoes.
Max Theiler successfully propagated yellow fever virus in chicken embryos and developed a live attenuated vaccine (strain 17D) that remains in use today

Methods progressed from whole organ cultures to primary cell cultures and eventually to immortalized cell lines.
Enders and colleagues in 1949 successfully propagated poliovirus in primary human cells.
Renato Dulbecco developed the plaque assay, which allowed for more accurate quantification of animal viruses by counting plaques on cultures, determined the number of infectious virus particles.

In 1941, Hirst observed haemagglutination of red blood cells by influenza viruses which proved to be a useful tool.
In the 1960s and later, improved methods for detecting viruses were developed: complement fixation, radioimmunoassays, and immunofluorescence.
Enzyme-linked immunosorbent assay (ELISA) and Western blot assays are quantitative and sensitive.
Kohler and Milstein isolated the first monoclonal antibodies in 1975, enabling more precise study of virus proteins and wider use in virological assays for wider use in diagnostics.

Ernst Ruska invented the first electron microscope in 1931.
The electron microscope overcame the limitations of light microscopes regarding resolution, enabling the direct examination of viruses at magnifications exceeding 100,000x.
Transmission electron microscopy yields detailed information about virus structure.
The scanning electron microscope provides three-dimensional images.

Virions are the inert carriers of the viral genome, assembled in cells, but do not grow or divide.
Viruses vary in shape, including helical, icosahedral, enveloped, and complex forms.