Viruses are the smallest infectious agents and can infect all forms of life. They are obligate intracellular parasites that contain either RNA or DNA. Many important human viruses are described including influenza, measles, hepatitis B, and HIV. Viruses have a protein coat that protects their genetic material and use host cells to replicate. Host responses to viral infection include innate immune defenses and adaptive immune responses like antibodies and T cells. Acute viral infections are generally mild and short-lived while some viruses can establish persistent infections by becoming latent or causing chronic or latent infections.

1. “Knowledge is a process of piling up
facts; wisdom lies in their
simplification.”

2. VIRUSES: GENERAL PROPERTIES;
DISEASES, AND HOST RESPONSE.
``Probably the most common infectious agents``
Class of 2011 3rd
year
Prof. Abbas Hayat

3. • 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.

7. Some medically important viruses.DNA Viruses
Family Virus Disease
Poxvirus Variola,
molluscam
Smallpox,
molluscam
contagiosum
Herpes virus Herpes simplex,
vaicella zoster,
cytomegalo
virus, EB virus.
Herpes, chicken
pox, shingles,
infectious
mononuleosis.
Adenoviruses Adenoviruses Sore throat,
conjunctivitis,
G.I.T. infections.

8. Hepadnavirus Hepatitis B Hepatitis
Papoviruses Pappiloma,
polyoma, SV
40
Warts, progressive
multifocal
leucoencephalopathy
Parvoviruses B19 Erythema infectiosum,
hemolytic crisis.

9. Some medically important RNA viruses
Orthomyxovirus Influenza Influenza
Paramyxoviruses Para influenza,
Respiratory
syncytial,
measles, mumps
Respiratory inf.
Measles, mumps.
Rhabdoviruses Rabies Rabies
Picornaviruses Enteroviruses,
rhinoviruses,
Meningitis,
paralysis, colds,

10. Flaviviruses Flavivirus Encephalitis,
febrile disease.
Bunyaviruses Buniyaviruses,
Hantan virus
Encephalitis,
febrile disease
Reoviruses Rotavirus gastroenteritis
Arenaviruses Lynphocytic
choriomeningitis,
Lassa virus.
Meningitis ,
febrile disease
Retroviruses HTLV -I, II,
HIV -I, II
T cell leukemia,-
lymphoma, AIDS

11. • 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
sheep

12. 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;

13. No active Ribosome's or protein
synthesizing equipment
Some have enzymes within their
particle.
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.

15. STRUCTURE OF VIRUSES
• Virus basically consists of a core of
nucleic acid surrounded by a
protein coat.
• 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

16. 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.

18. 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

19. CUBIC SYMMETRY

20. ICOSOHEDRAL & HELICAL SYMMETRY

22. THREE METHODS OF
CULTIVATION
• 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
Monkeys

23. 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
cell
3. Latent Infection . The virus remains within
the cell in a potentially active state but
produces no obvious effect on the cells
function.

26. THREE METHODS OF PENETRATION

30. 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
recovery.
2. Many infections are silent
and the virus multiplies in
the body without causing
any symptoms.
3. Few are severe and always

35. Host responses to viral
infection:
SPECIFIC IMMUNE
RESPONSES
(Adaptive Immunity)
NON SPECIFIC
DEFENSES
(Innate Immunity)
1. Neutralizing antibody
response
1. Interferon - alpha-
interferon mainly,
stimulated in
surrounding cells
2. Cell-mediated
responses (i.e.,
cytotoxic T
lymphocytes)
2. Body fluids -
stomach acid, tears
3. Complement-
mediated lysis
3. Mechanical barriers
- respiratory

38. Acute infections:
• 1.Hit and run infections.
Characterized by the
common cold viruses
(rhinoviruses).
• 2. Short incubation period
(36 to 48 hours).

40. 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).

41. Systemic infections.
1. Characterized by the common childhood diseases
caused by measles, chickenpox, mumps, and
rubella.
2. Incubation period is long - 14 to 21 days. There
are two viremic phases during the incubation
period:
Replication at primary site ---- Primary viremia ----
replication in lymphoreticular system and viscera
---- Secondary viremia ---- target organs

49. Several types of persistent viral infections
exist:
• 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
and VZV).
• 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.

50. • 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

52. 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.``