Viruses are pathogens that cause disease in animals and plants. They are infectious agents too small to be seen with a light microscope. Viruses are not living cells and cannot replicate independently - they need a host cell to reproduce. Examples of viruses include influenza, polio, and dengue fever. The branch of science that studies viruses is called virology. Viruses have either DNA or RNA as their genetic material and range in size from 20-250nm. They are obligate intracellular parasites that rely on host cells for reproduction and can cause disease in the host.

1. VIRUSES

2. Definition:
The viruses are pathogens,which cause disease in
animals and plants.
These are infectious agents cause many diseases.
Examples:
Influenza
Bird flu
Polio
Swine flu
Dengue fever etc

3. Viruses are not cells.
Not capable of independent replication.
They cannot synthesize their own energy
and proteins.
They are too small to be seen under light
microscope

4. History And Discovery Of Virus
Charles Chamberland and Louis Pasteur:
Derived from a Latin word Venome meaning poison.
In the past the term virus was associated with
infectious diseases
which have unknown cause.
The first evidence about the existence of Virus came
when in
1884 Charles Chamberland worked with Louis
Pasteur found
that The causative agents of rabies(disease due to

5. However such filters could be used to
completely remove all bacteria or other cells
known at that time from liquid suspension.

6. Iwanowsky(1892):
Tobacco mosaic disease was thought to be caused by
bacteria.
Iwanowsky extracted the juice from the leaves of tobacco
having tobacco mosaic disease .
In order to remove bacteria the juice was passed through
porcelain filter.
He then rubbed the filtered juice on the leaves of healthy
plants ,expecting non disease to develop,but the healthy
leaves soon showed the symptoms of the disease.

7. W.M.Stanley(1935):
By 1900,similar disease producing substance had been
discovered in both plants and animals.
The name filterable viruses were given to these
substances.
Stanley crystallized the infectious particle ,now known as
tobacco mosaic virus (TMV).
Subsequently many other viruses actually have been seen
with the help of electron microscope.

8. Virology
The branch which deals with the study of
viruses is called virology.

9. Characteristics of Virus
Viruses are small infectious agents and can be seen
under electron microscope.They have following
characteristics:
Size
They range in size from 250nm of poxviruses to 20nm
of parvoviruses.
Filterable:
They are 10 to 1000 times smaller than bacteria.So they
can pass through the pores of filter paper from which
bacteria cannot pass.

10. Obligate intracellular parasite:
Viruses cannot grow on artificial media.They can
reproduce in animal cells,plant cells or in
microorganisms.Here they reproduce by replication (a
process by which many copies or replicas of virus are
formed).Therefore viruses are obligate/strict intracellular
parasites (as they cannot grow/reproduce without host)
Disease production(Pathogenic):
They can cause diseases in the host during reproduction.
Resistant to antibiotics:
They are generally resistant to many antibiotics such as
penicillin,streptomycin and others.

11. No metabolic machinery:
Viruses have no metabolic machinery for the synthesis of
their nucleic acid and proteins.They depend on host cell to
complete these functions.
Living characteristics:
1. Viruses occur in different varieties or strains.
2. They have their own genetic material in the form of either
DNA or RNA that can undergo mutation.
3. They reproduce using the metabolic machinery of the
host cell they infect.
4. They enter the cells of living organisms and cause
diseases i.e intracellular obligate parasite.
5. They are destroyed by ultraviolet rays.

12. Nonliving characteristics:
1. They lack cellular structure,coenzymes and enzyme
system and don’t have metabolic activity of their own.
2. They can be crystallized and stored in bottles.
3. They don’t respire.
4. Viruses behave as nonliving ,inert infectious particles
outside the host.

13. General Structure of a Virus
Virion
The complete ,mature and infectious particle is called virion .
OR
A complete viral particle is called virion.
Parts of virus
Virus can be divided into two parts:
1. Core
2. Coat

14. CORE
It is inner part of virion which consists of Viral genome
And various proteins (enzymes).
GENOME
It is genetic material which is either DNA or RNA ,
which may be single stranded or double stranded.
CORE PROTEINS
Is includes one or more enzymes that facilitates the
virus in it’s mode of action with in host body ,
For example : All single stranded RNA viruses have the
enzyme to convert single stranded RNA genome into
double stranded RNA genome.

15. Retro viruses and hepatitis B virus contain the enzyme
reverse transcriptase to convert single RNA genome into
double stranded DNA genome.