VIRUS
Introduction:
Viruses are ultramicroscopic, acellular
infectious agents that can only replicate inside
living host cells. They are considered obligate
intracellular parasites because they lack the
machinery required for independent metabolism
and reproduction. Viruses infect all forms of life,
including humans, animals, plants, and even
bacteria (bacteriophages). They are important
medically because they cause a wide range of
diseases such as influenza, hepatitis, HIV/AIDS,
measles, and COVID-19.
SHAPE AND SIZE OF VIRUSES:
Feature Description Examples
Smaller than bacteria; seen only
Size range Very small, about 20–300 nm
under electron microscope
Icosahedral Cube-like symmetry, appears Adenovirus, Poliovirus, Herpes
(spherical) round under microscope virus
1
, Feature Description Examples
Spiral arrangement of nucleic
Helical (rod-like) Influenza virus, Rabies virus
acid and capsid
Combination of structures (head
Complex shape Bacteriophages (T4 phage)
and tail)
Enveloped Surrounded by lipid membrane
HIV, Influenza virus
viruses from host cell
Non-enveloped Only nucleocapsid present, no
Adenovirus, Poliovirus
viruses envelope
STRUCTURE OF VIRUS :
1. Introduction to Viral Structure:
Viruses are acellular
infectious agents that have a
very simple structure compared
to bacteria. They consist
mainly of genetic material and
a protective protein coat.
Some viruses also
possess an outer envelope.
Despite their simplicity,
their structure is highly
specialized for infection and
replication inside host cells.
2. Nucleic Acid Core (Genome):
The innermost part of a virus is
its genetic material, which may
be either DNA or RNA (never
both in a single virus).
This nucleic acid carries all the
genetic information required for
viral replication and protein
synthesis inside the host cell.
Viruses may contain single-
stranded or double-stranded
nucleic acid depending on the
type.
For example, HIV is an RNA
virus, while Herpes virus is a DNA virus.
2
Introduction:
Viruses are ultramicroscopic, acellular
infectious agents that can only replicate inside
living host cells. They are considered obligate
intracellular parasites because they lack the
machinery required for independent metabolism
and reproduction. Viruses infect all forms of life,
including humans, animals, plants, and even
bacteria (bacteriophages). They are important
medically because they cause a wide range of
diseases such as influenza, hepatitis, HIV/AIDS,
measles, and COVID-19.
SHAPE AND SIZE OF VIRUSES:
Feature Description Examples
Smaller than bacteria; seen only
Size range Very small, about 20–300 nm
under electron microscope
Icosahedral Cube-like symmetry, appears Adenovirus, Poliovirus, Herpes
(spherical) round under microscope virus
1
, Feature Description Examples
Spiral arrangement of nucleic
Helical (rod-like) Influenza virus, Rabies virus
acid and capsid
Combination of structures (head
Complex shape Bacteriophages (T4 phage)
and tail)
Enveloped Surrounded by lipid membrane
HIV, Influenza virus
viruses from host cell
Non-enveloped Only nucleocapsid present, no
Adenovirus, Poliovirus
viruses envelope
STRUCTURE OF VIRUS :
1. Introduction to Viral Structure:
Viruses are acellular
infectious agents that have a
very simple structure compared
to bacteria. They consist
mainly of genetic material and
a protective protein coat.
Some viruses also
possess an outer envelope.
Despite their simplicity,
their structure is highly
specialized for infection and
replication inside host cells.
2. Nucleic Acid Core (Genome):
The innermost part of a virus is
its genetic material, which may
be either DNA or RNA (never
both in a single virus).
This nucleic acid carries all the
genetic information required for
viral replication and protein
synthesis inside the host cell.
Viruses may contain single-
stranded or double-stranded
nucleic acid depending on the
type.
For example, HIV is an RNA
virus, while Herpes virus is a DNA virus.
2
