The word virus comes from Latin. It means poison. In old days anything that used to cause disease was called virus. Virus is known as an ultramicroscopic non-cellular disease creating particle. It cause many diseases to both plant and animals.
Virus have the following characteristics:
1. They are the smallest living organisms.
2. They do not have a cellular structure.
3. They can only reproduce by invading living cell. Therefore they are all parasitic. They are obligate endoparasites, meaning that they can only live parasitically inside other cells. Most are causes disease.
4. They have simple structure, consisting of a small piece of nucleic acid, either DNA or RNA, surrounded by a protein or lipoprotein coat.
5. They are on the boundary between what we regard as living and non-living.
6. Each type of virus will recognize and infect only certain types of cell. In other words, viruses are highly specific to their hosts.
Virus is ultra microscopic and it can not be seen without electron microscope. Generally the size of virus varies from 12 millimicrons (polio) to 300 millimicrons (Tobacco Mosaic). The Potato mosaic and cattle pox are larger in size.
Crystallized virus found to be of following shapes: Round, Rod Shaped, Tadpole shaped, Thread like, Bread like, Multiaxid etc.
1. On the basis of shape of virus
- Rod-Shaped : TMV, Alfalfa Mosic, Mumps
- Spherical : Polio, TIV, HIV, Dengue
- Cubical : Herpes, Vaccinia
- Tadpole shape : T2, T4, T6
- Oval Shape : Influenza
2. On the basis of host:
- Plant virus : TMV
- Animal virus : Influenza
- Bacterio-phage : T2 bacteriophage
- Cyano-phage : Blue green algae
- Avian Flue : H5N1/SARS
3. On the basis of Nucleic acid:
- DNA : T2, Vaccinia, TIV, Variola, Herpes simplex, Adeno, Caliphate
- RNA : Phio, Ralodo, Retro, Togo, Paramyno, Orthomysco, TMV, Potato X virus, Sugarcane mosaic, Turnip mosaic, Alfalfa mosaic, Polio, Dengue, Mumps, Measles, etc.
1) In preparing antidotes/vaccine: Pox, mumps, polio, jaundice etc diseases can be controlled by penetrating living or dead virus in human body as vaccines.
2) In controlling harmful animals and insects: Some animals and insects which are harmful for humans can be controlled by some special virus.
3) Control of disease: T2 bacteriophage saves humans from dysentery by spoiling some harmful bacteria like E.coli
4) In laboratory: They are used in lab, as the simplest living model. In the research of genetics virus is used. It is an important subject in genetic engineering.
5) In the evidence of evolution: Virus plays a key role to acquire knowledge about the trend of evolution, and the process of formation of living organisms because they contains both living and non-living characteristics.
Virus destroys plenty of bacteria useful for humans. Different diseases like common Cold, Influenza, Mumps, Pox, Polio, Yellow fever, Harpish, Hepatitis, Aids etc. are caused by the attack of virus.