What is Virus
The viruses are the simplest forms of life which instead of having the cellular organization (viz. plasma membrane, cytoplasm, and nucleus) similar to bacteria, blue-green algae, plants and animal, contain definite genetically determined macromolecular organization, genetic material and characteristic mode of inheritance.
Who discovered the virus? When Virus Discovered?
The viruses were first discovered by Iwanowski (1892) as extremely small micro organisms.
The viruses are sub-microscopic, acellular, forms of life and are much smaller than bacteria. They range in size from 20 to 80 m. Iwanowski showed that mosaic disease in the leaves of tobacco is caused by the ultramicroscopic agents that can pass even through the pores of fine filter paper which did not allow even bacterial cells to pass through.
The viruses possess a regular geometrical and macromolecular organization. Basically, all viruses consist of a core of only one type of nucleic acid (DNA/ RNA) which remains wrapped in a coat of protein called capsid. The capsid is composed of numerous protein molecules called capsomeres. The capsomeres determine the shape of the virus particle or viron, thus the viruses may contain three types of symmetry such as cubic (eg Bacteriophase x 174, Turnip yellow mosaic virus, etc.) Helical (eg Potato spindle tuber virus) have no capsid around the nucleic acid core or viral chromosomes. There are some highly specialized viruses such as influenza virus and mumps containing a membranous envelope around the capsid.
How does Virus work?
Viruses contain only one kind of nucleic acid as the hereditary material. The DNA viruses have single DNA molecule which may be either linear (having free and) or circular (having no free end) in shape, mostly, the linear DNA molecule is double stranded (eg P 22 bacteriophage). The circular DNA molecule may be either single stranded (eg x 174 bacteriophages) or double stranded as in most animal viruses. The RNA viruses are the only biological systems known in which RNA is the genetic material mostly, the RNA is present in its usual single stranded form (eg Plant viruses, Influenza etc.) but some viruses such as Retrovirus have a core of double stranded RNA similar in properties to DNA.
Why is virus host specific?
The viruses lack in necessary energy yielding and synthetic enzyme systems. Therefore, they cannot lead a free living mode of life for the performance of fundamental life activities such as reproduction, genetically determined structures and functions, they have to lead a parasitic mode of existence. All viruses are, therefore, invariably intercellular parasites of specific hosts which may be bacteria plants, or animals. The viruses may be classified into following 3 groups according to the type of host:
Types of Viruses
Bacterial viruses or Bacteriophages
Disease caused by Virus in Human
|Disease||Causative Pathogen||Mode of transmission||Incubation|
|Small Pox||Variola Virus||Direct contact (droplets), Indirected by infected articles||12 days|
|Chicken Pox||Varicella Virus||Direct contact (droplets), Indirect by Infected object||12-16 days|
|Common Cold||Rhinovirus||Contact||2-5 days|
|Influenza /Flu||Orthomixo-virus||Contact, Virus transmitted through discharge from respiratory tracts of persons infected with disease||1-2 days|
|Measles||Measles virus (Paramyxo virus)||Direct contact, virus transmitted through air by droplets during talking Coughing and sneezing||10-14 days|
|Mumps||Mumps virus||Direct contct, virus in Saliva and Secretin of nose invades salivary glands||12-21 days|
|Viral encephalitis||Encephalitis virus (arbovirus)||Some domestic animals reservoir of virus, transmitted by mosquito bite to man||4-21 days|
|Poliomylitis||Poliovirus||Contact, housflies, fleas, food and water||7-14 days|
|Rabies||Rabies virus||Bite of a mad (Rabid) dog||2-16 days|
|Dengue fever||Dengue virus||Mosquito (Aedes) bite||4-8 days|
|Herpes simplex||Herpes virus simplex||Contact, Saliva, stools, contaminated articles|
|Herpes Zoster||Herpes virus zoster||Contact droplets||7-14 days|
|Acquired immuno Deficiency Syndrome (AIDS)||Human T-cell Leukemia Virus (HTLV-III); also called LAV||Via blood and sperm among homosexuals, heterosexuals, intravenous drug users, hemophiliacs, promiscuous individuals and prosititutes||2 months to 10 years|