1. A virus is a small particle that contains genetic material and requires a host cell to replicate. It has a protein coat and in some cases an envelope. 2. Viruses are classified based on their morphology, genome, replication strategy, and other properties. The International Committee on Taxonomy of Viruses (ICTV) establishes standards for viral classification. 3. Viruses infect host cells by attaching to receptors on the cell, entering, hijacking the cell's machinery to produce more viral components, which then assemble and exit the cell to infect other cells.

1. VIROLOGY
• A virus is a particle very small in size
(25nm-500nm) that is filterable
totally dependent upon a living cell
for replication and existence i.e lack
Lippman’s system for replication
• A set of one or more nucleic acid
template molecule(s), normally
encased in a protective coat or coats
of protein or lipoprotein.
• Basically a bundle of genetic material
either DNA or RNA carried in a shell
called the viral coat or capsid which
is made of protein segments called
capsomeres
• Some viruses have additional layers
around the coat called envelope
• Viruses exhibit characteristics of
living things only when in a living
system
General Properties
1. Sub-microscopic particles
2. Filterable
3. Intracellular obligate
4. Dried and crystallized for storage
5. Mono-nucleic acid particles
6. Depend on host for amino acids
7. Specific to hostcells

2. Distinctive features of viruses
Lack continuous membrane
separating them from the host cells
Absence of protein synthesis systems
Replication is by synthesis of a pool
of components followed by assembly
of many viral components from the
pool in the host cell
CLASSIFICATION OF VIRUSES
• Attempt to classify viruses was
started by Holmes in 1957.
• He used symptoms
(symptomatology) as the only criteria
For example:
Enteric-influenza, hepatitis, polio
Respiratory-adeno, influenza,
measles
CNS- polio, measles
Reproductive
• Explain why this criteria is erroneous
ICTV Classification
• ICTV was set up in 1965 and
formulated definition of viruses and
their classification
Criteria established by ICTV
1. Morphological characteristics

3. 2. Genome properties
3. Protein properties
4. Replication strategy
5. Site of accumulation
6. Cytopathology
7. Biological properties
8. Serological properties
MORPHOLOGICAL CHARACTERISTICS
• Shape-using E.M the shape of the
virus can be observed like: helical,
polyhedral/icosahedral, complex,
block, enveloped, bullet,
pleomorphic
2. Genome Properties
• Nucleic acid type- DNA or RNA
• No of strands of nucleic acid(double
or Single) and their physical
construction (linear or
circular/segments; polarity)
• Size of genomes in kbp
3. Replication Strategy
• Negative sense RNA
• Positive sense RNA
4. Cytopathology-effects of the virus at
cellular level (lytic or non-lytic)
5. Site of accumulation-
cytoplasm/nucleus
6. Biological properties

4. MODES OF VIRAL TRANSMISSION
1. Mechanical Transmission
To cause a disease a virus must find a
portal of entry which provides access
to the cells in which it can multiply
Mechanical transmission occur by
direct contact with contaminated
food or water; inhalation of droplets
with the virus.
Some of the viruses spread through
this mechanism include:
picornaviruses, adenoviruses,
myxoviruses
2. Vector Transmission
The virus is moved from one
organism to another by a vector
Usually the virus is vector species
specific e.g yellow fever virus uses
mosquito Aedes species
Eradicating the vector controls the
spread of the virus
Patterns of human viral transmission
a. Human to human transmission
• The virus is contained in a small
human population acting as carriers
from where it is spread to others e.g
the measles virus
• This type of virus maintain a
minimum cycle in the population
b. Animals to humans
• Animals act as reservoirs for the virus
from where it is passed to human

5. c. Vector to vertebrates to humans
• The maintenance cycle is in the
vectors which pass the virus to
vertebrates which interact with
humans passing it over
• Humans are susceptible and seem to
be the dead-end host.
• Example St’ Louis encephalitis
Pathogenesis of Viral infections
• This is the origination and
development of a disease
• Viral infections can be acute (short
term incubation), chronic(stay longer
in the host) and latent/persistent
• The first step in the disease process
is exposure
Exposure and Transmission
This may occur through direct
contact , indirect contact or vectors
Transmission of virus from mother to
offspring can occur through
transplacental, perinatal or colostral
and this is referred as vertical
transmission
Transmission through other routes is
referred to as horizontal
Portal of entry
Viruses enter the host through:
a. Respiratory tract (by aerosolized
droplets)

6. b. Alimentary canal (oral-faecal
contamination)
c. Genitourinary tract
d. Conjunctivae
e. Breaches on the skin (abrasions,
needles and insect bites)
• For infection to occur after entry the
virus must be able to initiate it in
susceptible cells
• Susceptibility of cells to a given virus
depends largely on their surface
receptors which allow for attachment
and subsequent penetration of the
virus.
• Once the virus penetrates into host
cells it can cause localized or
disseminated infections
• Localized-the virus replicates at the
point of entry and causes infection at
that point
• Disseminated or systemic-the virus
spreads from the point of entry to
other organs causing infections far
from point of entry
• Examples: Porcine teschovirus type 1
Factors influencing viral infections
1. Pre-existing immunity
2. Genetics of the animal
3. Age of the animal
4. Stress related factors etc

7. • Viruses cause diseases through direct
effects on cells (like death, CPE or
malignant transformation) or indirect
effects by the immunological and
physiological responses e.g rotavirus
infection where the infected
erythrocytes produce cytokines
which excite the enteric neurons
inducing secretions of excess fluids
and electrolytes into the large
intestines
NOTE
Pathogenicity is the ability of a virus
to cause a disease while virulence is
the degree/level of pathogenicity
Avirulent virus lacks a bility to cause
infection while attenuated virus is one
that has been weakened frequently by
multiple passages in cell cultures to
reduce its virulence.
How do viruses evade the immune
system of the host??? Discuss this
question and attach it as a pdf
document for me to mark.

8. VIRAL MULTIPLICATION/REPLICATION
• Viruses are capable of causing
infection only when there is
multiplication i.e No multiplication
No infection
• Pathological effect of the virus
irrespective of the host is an interplay
of three factors:
1. Toxic effects of viral products
2. Reaction of the host cell to the
infecting virus
3. Modification of host gene
expression by structural or
functional interaction with genome
of the virus
• Viruses depend on the synthetic
machinery of the host cell for
replication because of lack of
biosynthetic enzymes
• The replicative cycle can be divided
into five merging steps:
i. Adsorption/attachment
ii. Penetration
iii. Biosynthesis
iv. Maturation
v. Release
Adsorption/Attachment
The virus attaches/adsorbs at a
particular site on the host cell which
is called a receptor.

9. • Attachment involves electrostatic
bonds between the viral proteins
and the receptors (lipoproteins or
glycoproteins on cell surfaces).
• Adsorption is specific and mediated
by binding of virion surface
structure/proteins/legands (e.g
gp120 on HIV) to receptors (E.G CD4
60KD glycoprotein on the surface of
mature T lymphocytes.
Penetration
• Viruses uses any of the following
mechanisms to enter host cells:
Direct passage-after attachment to
the cell membrane the virus passes
Directly to cells without forming
phagocytic vacuole. Once in the
cytoplasm the capsid is removed by
cellular proteolytic enzymes and
nucleic acid released.
Fusion-some enveloped viruses
enter the cell by fusion of the viral
envelope with the cell membrane
Endocytosis or viropexis-
predominant entry mechanism
among viruses. The viral particle
enters the cell in avacuole or vesicle
Biosynthesis
• Synthesis of viral nucleic acid,
proteins and capsid protein.

10. • This process follows the steps below:
a. Transcription of mRNA from viral
nucleic acid
b. Translation of mRNA into eproteins
(enzymes)
c. Replication of viral nucleic acid
d. Synthesis of late proteins-
components of the virus (capsid)
Maturation
• Period at which viral particles
accumulate in the cell
• Assembly of viral particles follows
synthesis of viral nucleic acid and
proteins in the nucleus or cytoplasm
• Enveloped viruses acquire the
envelope from the cell membrane of
the host during budding.
Release
• Occurs through lysis or exocytosis
• The whole cycle takes 15-30 hrs in
animal viruses
pfu/
ml
latency
hours after adsorption