Viruses are still biologists’ puzzle because they show both living and nonliving characters. Hence viruses are regarded as a separate entity. It is not taken into account in Whittaker’s five kingdom classification. Viruses are now defined as ultramicroscopic, disease causing intra cellular obligate parasites.

Brief history of discovery
Viruses were not known to biologists for a long time due to their ultramicroscopic structure though their presence was apparent by infectious diseases which were proved not due to bacteria. It attracted the attention of investigators only in the 19th century when a virus called tobacco mosaic virus (TMV) caused severe damage to commercially important tobacco crop.

Enigma of Viruses

Mayer demonstrated that the disease could be transmitted just by applying the sap of infected leaf to the leaf of healthy plant. He thought that the disease was due to a bacterium. It was then the Russian biologist Iwanowsky (1892) who demonstrated that the sap of infected leaves even after passing through bacterial filter remained infective, ruling out the bacterium as the causative agent. Dutch microbiologist Beijerinck (1898) confirmed the findings of Iwanowsky and called the fluid “contagium vivum fluidum” which means contagious living fluid. This was later on called virion (poison) and the disease causing agent as virus. W.M. Stanley (1935), the American biochemist, isolated virus in crystalline form and demonstrated that even in that state it maintained the infectivity. This marked the beginning of a new branch of science called virology.

General characteristics
Viruses are ultramicroscopic and can cause diseases in plants and animals. They are very simple in their structure. They are composed of nucleic acid surrounded by a protein coat. Nucleic acid can be either RNA or DNA, but never both. They have no cellular organization and have no machinery for any metabolic activity. They are obligate intracellular parasites and they multiply within their host cells. Once outside the host cell they are completely inactive.

Size and Shape
Viruses are very minute particles that they can be seen only under electron microscope. They are measured in millimicrons ( 1 millimicron = 1/1000micron). (1micron – 1/1000 millimeter). Generally they vary from 2.0 mm to 300 mm in size. Very small size and ability to pass through bacterial filters are classic attributes of viruses.

The following methods are used to determine the size of the viruses.

1. Direct observation by using electron microscope

2. Filtration through membranes of graded porosity: In this method viruses are made to pass through a series of membranes of known pore size, the approximate size of any virus can be measured by determining which membrane allows the virus to pass through and which membrane holds it back.

3. Sedimentation by ultra centrifugation : The relationship between the size and shape of a particle and its rate of sedimentation permits determination of particle size.

4. Comparative measurements: The following data is used for reference.
a. Staphylococcus has a diameter of 1000 mm.
b. Bacteriophage varies in size from 10-100 nm.

Shapes of Viruses:

Different shapes of Viruses

1. Cubic symmetry: polyhedral or spherical – eg. Adeno virus, HIV

2. Helical symmetry: e.g. Tobacco Mosaic virus (TMV), Influenza virus.

3. Complex or atypical: e.g. Bacteriophage, Pox virus

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