General properties of virus - Medical Virology

 

v  Viruses are the smallest infectious particles, first described as the “filterable agents” discovered in 1898.

v  Russian bacteriologist Dimitri Iwanowski filtered the sap of diseased plants through a porcelain filter that was designed to retain bacteria.

v  Their small size allows them to pass through filters designed to retain bacteria.

v  Martinus Willem Beijerinck called the filtered, infectious substance a “virus”, calling his discovery a 'contagium vivum fluidum'

v  Martinus Beijerinck is often called the Father of Virology.

v  1904–08 V. Ellermann, O. Bang, H. Vallee, H. Carre first demonstrated a leukemia-causing retrovirus.

v  Viruses are not living, obligate intracellular parasites, requiring host cells for replication.

v  Ranging in diameter from 18 to 1500 nanometres.

v  Requires Electron microscope for visualization, cannot be seen with a light microscope.

v  Viruses typically contain either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but not both

v  Some viral-like particles do not contain any detectable nucleic acids (e.g., prions),

v  The viral nucleic acids required for replication are enclosed in a protein shell (capsid) with or without a lipid membrane coat.

v  The cells they infect and the host response to the infection dictate the nature of the clinical manifestation.

v  Viruses lack the capacity to make energy or substrates, cannot make their own proteins, and cannot replicate their genome independently of the host cell.

v  To use the cell’s biosynthetic machinery, the virus must be adapted to the biochemical rules of the cell (viral messenger RNA, protein, and identical copies of the genome).

v  The physical structure and genetics of viruses have been optimized by mutation and selection to infect humans and other hosts.

v  Viruses have a naked capsid or an envelope morphology.

v  Viral components are assembled and do not replicate by “division.”

v  To cause infection in humans the virus must be capable of transmission through potentially harsh environmental conditions

o   must traverse the skin or other protective barriers of the host,

o   must be adapted to the biochemical machinery of the host cell for replication, and

o   must escape elimination by the host immune response.

v  More than 2000 species of viruses have been described, with approximately 650 infecting humans and animals.

v  Infection can lead either to rapid replication and destruction of the cell or to a long-term chronic relationship with possible integration of the viral genetic information into the host genome.

v  Diseases caused by viruses in human beings fall into a wide spectrum.

o   Certain diseases carry invariably a fatal outcome - Rabies and Acquired Immunodeficiency Syndrome (AIDS).

o   Some of the viral diseases such as Hepatitis, Encephalitis, Ebola Haemorrhagic Fever and Yellow Fever frequently turn out to be fatal.

o   Tremendous morbidity results from worldwide prevalent diseases such as measles, mumps, rubella, common cold, influenza and chickenpox.

o   Some of these usually appear in epidemic forms (e.g. measles) and some may even acquire pandemicity (e.g. influenza).

o   Rubella virus is well known to induce teratogenic effects and evidence is accumulating at a rapid pace to incriminate viruses as carcinogenic in human beings.

v  Host range of Viral infection varies from microscopic bacteria to macroscopic animals., i.e.. a virus can infect all forms on earth - invertebrates, vertebrates, plants, protists, fungi, and bacteria.

v  Nomenclature of viruses is decided by the International Committee on Taxonomy of Viruses.

v  Viruses are grouped into families which are named with a suffix-viridae. Subfamilies shall end with suffix-virinae and the genera will have suffix of -virus.

v  The prefix may be another latin word or a sigla, i.e. an abbreviation derived from some initial letters.

The latinized names are written in italics whereas vernacular names are written in roman letters.