The word virus is derived from the Latin word ‘venome’ which means poisonous fluid. Viruses are Non-cellular, submicroscopic infectious agents which contain either RNA or DNA, are enclosed by the proteinaceous coat, and reproduce only in living cells (obligate intracellularly: parasites) Viruses use the biosynthetic machinery of the host to make their materials and then transfer them to their cells. The study of viruses is known as virology.
Nearpeer biology Notes
Historical Background of Virus
Some viral diseases have been known for centuries. The first infectious disease against which presentation was developed was a viral disease.
Work of Edward Jenner: In 1796. Edward Jenner discovered an effective method for the prevention of the viral disease smallpox. He removed material from a cowpox lesion on the hand of a milkmaid and injected it into an 8 years old boy (James Phipps). After six weeks the boy was injected with pus from a smallpox victim. He did not develop the disease. Jenner used material for vaccination from cowpox lesions and successfully vaccinated 23 persons. As the material was obtained from cows (called Vacca in Latin), this method was named vaccination by Louis Pasteur.
Work of Charles Chamberland: Charles Chamberland (1884) found that bacteria cannot pass through porcelain filters. However, agents responsible for rabies can pass through these filters. Any toxic substance that caused disease was called a virus. These unseen filterable agents of rabies were called filterable viruses. Rabies is a disease that is transferred to humans by bites from rabid dogs, foxes, cats, bats, and other animals.
Work of Ivanowski: In, 1892, Levandowski discovered that the agent which caused tobacco mosaic disease was filterable. He obtained bacteria-free filtrate from infected plants and placed it on healthy leaves of tobacco. The filtrate caused the disease in healthy plants. Later these ultramicroscopic agents were also observed in victims of many diseases including foot and mouth disease (1898) and yellow fever (1901). In 1898 the Dutchman Beijerink formed the name ‘virus’ (Latin for poison) to describe the infectious nature of certain filtered plant fluids. Although progress was made in isolating highly purified samples of viruses and identifying them chemically as nucleoproteins (nucleic acids combined with proteins). the particles still proved mysterious because they were too small to be seen with the light microscope. They were among the first biological structures to be studied when the electret, microscope was developed in the 1930s. Stanley (1935) crystallized the tobacco mosaic virus.
Characteristics of Viruses Viruses are small infectious agents and can be seen under an electron microscope. They nave following characteristics:
Size: They range in size from 250 nm of Pox viruses to 20 nm of Parvoviruses.
Filterable: They are 10 to 1000 times smaller than bacteria. So they can pass through the pores of the filter from which bacteria cannot pass.
Obligate Intracellular Parasites: Viruses cannot grow on artificial media. They can reproduce in animal cells, plant cells, or microorganisms. Here they reproduce by replication (a process by which many copies or replicas of a virus are formed). Therefore, the viruses are obligate intracellular parasites.
No Metabolic Machinery: Viruses have no metabolic machinery for the synthesis of their nucleic acid and protein They depend on the host cell to complete vital functions.
Disease Production: They can cause disease in the host during reproduction.
Resistant to Antibiotics: They are generally resistant to many antibiotics such as penicillin, streptomycin, and others. Each type of virus will recognize and infect only certain types of cells. In other words, viruses are highly specific to their hosts.