One hundred years ago, researchers could not imagine submicroscopic
particles, and thus they described the infectious agent
as contagium vivum fluidum- a contagious flu id. By the 1930s
scientists had begun using the word virus, the Latin word for
poison, to describe these filterable agents. The nature of viruses,
however, remained elusive until 1935, when Wendell Stanley, an
American chemist, isolated tobacco mosaic virus, making it possible
for the first time to carry out chemical and structural studies
on a purified virus. At about the same time, the invention of the
electron microscope made it possible to see viruses.

The question of whether viruses are living organisms has an
ambiguous answer. Life can be defined as a complex set of
processes resulting from the actions of proteins specified by
nucleic acids. The nucleic acids of living cells are in action all the
time. Because viruses are inert outside living host cells, in this
sense they are not considered to be living organisms. However,
once viruses enter a host cell, the viral  nucleic acids become
active, and viral multiplication results. In this sense, viruses are
alive when they multiply in the host cells they infect. From a clinical
point of view, viruses can be considered alive because they
cause infection and disease, just as pathogenic bacteria, fungi,
and protozoa do. Depending on one's viewpoint, a virus may be
regarded as an exceptionally complex aggregation of non living
chemicals, or as an exceptionally simple living microorganism.

How, then, do we define virus? Viruses were originally distinguished
from other infectious agents because they are especially
small (filterable) and because they are obligatory intracellular
parasites-that is, they absolutely require living host cells in
order to multiply. However, both of these properties are shared
by certain small bacteria, such as some rickettsias.

The truly distinctive features of viruses are now known
to relate to their simple structural organization and their mechanism
of multiplication . Accordingly, viruses are entities that:
    Contain a single type of nucleic acid, either DNA or RNA.
    Contain a protein coat (sometimes itself enclosed by an
    envelope of lipids, proteins, and carbohydrates) that
    surrounds the nucleic acid.

    Multiply inside living cells by using the synthesizing
    machinery of the cell.
     Cause the synthesis of specialized structures that can transfer the viral nucleic acids to other cells.

    Viruses have few or no enzymes of their own for metabolism;
    for example, they lack enzymes for protein synthesis and ATP generation
    eration . To multiply, viruses must take over the metabolic machinery
    of the host cell. This fact has considerable medical significance
    for the development of antiviral drugs, because most drugs that would interfere with viral multiplication would also interfere with the functioning of the host cell and therefore are too toxic for clinical use.

    Cited By Kamal Singh Khadka
    Msc Microbiology,  TU


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