Text books have been written on virology and one example of the victory of humans on an infectious disease is small ox, usually infection of child hood, can be mild to very deadly disease.Just a very brief and simple introduction to viral diseases in one small lecture is an impossible task to do. Which I will attempt by over simplyfying, and keeping the basic gacts intact, the basic core concepts, doing injustice to such important infectious agents.
Text book on virology have been written , a important example of human conquering one disease fully has been small pox,
A regular icosahedron has 60 rotational (or orientation-preserving) symmetries, and a symmetry order of 120 including transformations that combine a reflection and a rotation. A regular dodecahedron has the same set of symmetries, since it is the dual of the icosahedron. The set of orientation-preserving symmetries forms a group referred to as A5 (the alternating group on 5 letters), and the full symmetry group (including reflections) is the product A5 × C2. The latter group is also known as the Coxeter group H3
Some virus leave tell tale signs or cause cell death which can be morphologically observed under the microscope in tissue culture or of different affected organs.
Introduction to virology
“Knowledge is a process of piling up
facts; wisdom lies in their
VIRUSES: GENERAL PROPERTIES;
DISEASES, AND HOST RESPONSE.
``Probably the most common infectious agents``
Class of 2011 3rd
Prof. Abbas Hayat
• Definition :
1. Viruses are the smallest infective
agents. Most form of life—not only
humans animals and plants, but
bacteria also are susceptible to
infection with appropriate viruses.
2. Obligate intracellular parasites
3. Have only RNA or DNA. A imp.
distinguishing factor from bacteria and
other infectious agents.
Some medically important RNA viruses
Orthomyxovirus Influenza Influenza
Paramyxoviruses Para influenza,
Rhabdoviruses Rabies Rabies
Flaviviruses Flavivirus Encephalitis,
Reoviruses Rotavirus gastroenteritis
Retroviruses HTLV -I, II,
HIV -I, II
T cell leukemia,-
• Virus like particles, or incomplete viruses.
• Bacteriophage: bacteriophage
from 'bacteria' and Greek φ γε νᾰ ῖ phagein "to eat") is any
one of a number of viruses that infect
bacteria. Bacteriophages are among the
most common biological entities on Earth
• Plasmid: DNA molecule that is separate from, and can replicate
independently of, the chromosomal DNA. Are double-stranded and,
in many cases, circular. Plasmids usually occur naturally in bacteria,
but are sometimes found in eukaryotic organisms
• Prion : infectious particles composed of protein and no
detectable nucleic acid or envelope. cause of certain slow diseases
like Creutzfeldt-Jacob Disease (CJD) in human and scrapie in
Three main properties :
1. Small size. 10nm- 300nm.
Bacteria are approx.
1000nm and R.B.C 7500nm.
2. Genome . DNA or RNA
3. Metabolically inert . No
metabolic activity outside
susceptible host cells;
No active Ribosome's or protein
Some have enzymes within their
Cannot multiply in inanimate media
but only in living cells.
On entry into susceptible cell, viral
genome or nucleic acid is
transcribed into ----or itself acts as
--------virus specific messenger or
mRNA which then directs the
replication of new virus particles.
STRUCTURE OF VIRUSES
• Virus basically consists of a core of
nucleic acid surrounded by a
• Protein coat protects the viral
genome from inactivation by
adverse environment factors e.g.
nucleases in the blood stream. It is
antigenic and often responsible for
stimulating the production of
The structure which makes up a virus particle
are known as :
• VIRION : Intact virus particle.
• Capsid : The protein coat .
• Capsomere : the protein structural units of
which the Capsid is composed.
• Nucleic acid : RNA or DNA
• Envelope : particles of many viruses
surrounded by a lipoprotein envelope,
containing viral antigens, but also partly
derived from the plasma or, in some cases the
nuclear membrane of the host cell.
THREE TYPES OF SYMMETRY
• 1. Cubic. in which particle are
icosohedral protein shells with nucleic
acid contained inside.
• 2. Helical. In which the particles
contains an elongated nucleocpsid ;
the capsomere are arranged round the
spiral of nucleic acid. Most helical
viruses possess an outer envelope.
• 3. Complex. Does not confirm to any
THREE METHODS OF
• Tissue Culture . Cells obtained from man
or animal are grown in artificial culture in
glass vessels in the Lab, Cells are living and
metabolizing and can support viral replication.
• Chick Embryo. Fertile chick eggs are used.
• Lab. Animals. Mice Rabbits Ferrets and
THREE EFFECTS OF VIRUSES ON CELLS
1. Cell Death . The infection is lethal : it causes a
cytopathic effect (CPE) which kills the cells.
2. Cell Transformation. The cell is transformed
from a normal to a malignant or cancerous
3. Latent Infection . The virus remains within
the cell in a potentially active state but
produces no obvious effect on the cells
THREE TYPES OF VIRAL DISEASES
Viruses are imp. and common
cause of human disease
especially in children.
1. Most are mild and patients
makes a complete
2. Many infections are silent
and the virus multiplies in
the body without causing
3. Few are severe and always
Host responses to viral
1. Neutralizing antibody
1. Interferon - alpha-
2. Body fluids -
stomach acid, tears
3. Mechanical barriers
• 1.Hit and run infections.
Characterized by the
common cold viruses
• 2. Short incubation period
(36 to 48 hours).
Pathology in the upper respiratory tract due to epithelial
cell killing and secondary invasion by normal flora of
oral bacteria. Symptoms last for 5-7 days eliminated
by the host.
Protection not good since there are >1000 serotypes of
rhinovirus. orthomyxoviruses, which include influenza
A and B.
Protection against subsequent infection may be poor
because these viruses can change their antigenic
profile of their surface proteins, e.g. hemagglutinin
(HA) and neuraminidase (NA), by either point
mutations (drift), or recombination, which results in a
completely different gene (shift). (Influenza viruses are
capable of antigenic shift because they have
segmented single-stranded RNA genomes).
1. Characterized by the common childhood diseases
caused by measles, chickenpox, mumps, and
2. Incubation period is long - 14 to 21 days. There
are two viremic phases during the incubation
Replication at primary site ---- Primary viremia ----
replication in lymphoreticular system and viscera
---- Secondary viremia ---- target organs
Several types of persistent viral infections
• Latent infections: After an acute cytocidal stage,
the virus becomes latent.
• The virus when latent does not replicate ,
produces limited mRNA. Latency may occur in
non-dividing cells such as neurons (HSV I, HSV-II
• Chronic infections: Persistence of virus after
acute phase with active replication of virus. May
cause disease in normal individuals, e.g. hepatitis
B, or not, e.g. JC and EBV.
• Chronic infection with neoplasia: Transformation
of virus infected cells usually after years of
persistence, e.g. HTLV-I , HTLV-II, hepatitis B,
Hep C, and HPV 16 and 18.
• The outcome can change depending on
the state of the individual, i.e. whether
they become immunocompromised.
• The site of viral persistence is
important as it would be difficult to
remain completely latent or with
minimal viral transcription and
translation in rapidly dividing cells such
as those lining the small intestine.
• Viral persistence requires viruses to develop
ways to evade immune responses. Herpes
Thank you & Remember
Keep on alert for viruses around you, they can
1. transform you
2. have cyto- pathic effects on you
3. Stay latent inside you…………….
``And you can do nothing much about them…..so
relax and keep going as best as you can.``