This document provides an introduction to virology. It defines viruses and their structure. Viruses are submicroscopic obligate intracellular parasites that contain genetic material surrounded by a protein coat. They can only replicate inside living cells. The complete virus particle is called a virion. Viruses are too small to be seen by light microscopes and have a fixed morphology like helical, polyhedral or spherical shapes. They contain either DNA or RNA and have a protein capsid that may contain an envelope. Viruses must infect host cells to replicate since they lack their own metabolic functions.

1. INTRODUCTION TO
VIROLOGY
KIMAIGA H.O
MBChB (University of Nairobi)

2. Virology
Virology is the studying of viruses.
A virus is a submicroscopic obligate (bound)
intracellular parasite containing genetic material/
single nucleic acid surrounded by a protein coat
and capable of replication only in the living cells
of humans, animals, plants(e.g. Tobacco Mosaic
virus), fungi, protozoa, algae, and bacterial cells
(bacteriophages). (No organism is safe from viral
infections)

3. Outside of living cells, viruses are
metabolically inactive hence are unable to
make their own proteins and essential
enzymes thus depend on their host cell for
energy and replication.
The complete infectious virus particle is
called virion and the genome is in a core
enclosed by the capsid.
Some viruses – called oncogenic viruses or
oncoviruses- cause specific types of cancer,
including human cancers such as
lymphomas, carcinomas, and certain types
of leukemia.

4. Viral Structure
Viruses are too small in size, between 10-300nm (.01-
0.03µrn)
20nm-Parvovirus
400nm-Poxvirus
Most viruses cannot be seen with light microscope
hence electronic microscopes are used
Virus particle is called a virion
All viruses consist of either single or double stranded
DNA or RNA surrounded by protective protein coat
called capsid which is usually symmetrical
Some viruses may have an envelope.
Capsid + genome = nucleocapsid

5. Virion structure
Lipid envelope
Spike projections
Protein capsid
Virion associated
polymerase
Nucleic acid

6. Viruses Properties
1. Every virus contains only one type of nucleic acid- either DNA
or RNA
2. Viruses must invade host cells to replicate.
3. Unlike cells, they do not divide by binary fission, mitosis, or
meiosis.
4. They lack the genes and enzymes necessary for energy
production.
5. They depend on the ribosomes, enzymes, and metabolites of
the host cell for protein and nucleic acid production.
6. Most viruses consist merely of nucleic acid surrounded by a
protein coat (capsid).
7. Viruses can be seen only by electron microscopes and each
virus have a fixed morphology (Helical, Polyhedral, Spherical,
or complex)
8. Viruses can infect bacterial cells (bacteriophages)

7. Differences between bacteria and
viruses
VIRUSES
Obligate intracellular parasites
No ribosomes
DNA or RNA (not both)
Seen by EM
10s-100s of genes
Tangled phylogeny
Does not contain protein
synthesis material
Arenoviruses – can undergo
protein synthesis on their own.
Only exception
Cannot be cultured/grown
Not sensitive to antibiotics
BACTERIA
Usually free-living but can
be parasites
Have ribosomes
DNA and RNA
Seen by LM
100s -1000s of genes
Natural phylogeny
Contains protein synthesis
material
Can grow on artificial
medium

8. Naming of viruses
Based on the disease they cause
e.g Poliovirus, rabies virus
Type of disease
e.g Marine Leukemia virus
Geographical locations
e.g Sendai virus, Coxsackie virus
Their discoverers
e.g Epstein- Barr virus
How they were originally thought to be contracted
e.g Dengue virus(evil spirit), influenza virus(influence of bad air)
Combinations of the above
e.g. Rous Sarcoma virus

10. Symmetry of Nucleocapsid
1. Helical.
 Rod-like with the RNA in the center of
the Helix
 A helix is made by stacking repeating
units in a spiral
 E.g.California Encephalitis Virus,
Coronavirus, Hantavirus, Influenza
virus(Flu Virus), Mumps virus,
Measles virus(Rubeola),
Parainflueanza virus,Rabies virus,
Respiratory Syncystial Virus(RSV).

11. 2. Icosahedral/ Cubic
 Many sided, 20 triangular faces and has 2-fold, 3-fold
and 5-fold symmetry axes
 The DNA or RNA is found at the centre of the capsid
 Preferred in virus since it contains more genetic
material hence faster replication
 E.g. Adenovirus(Causes common cold and pink-eye.
Has fibre structures extending off the capsid which help
them attach to cell surface structures)

12. Adeno-associated Virus (AAV)
Adenovirus
B19
Coxsackievirus - A
Coxsackievirus - B
Cytomegalovirus (CMV)
Eastern Equine Encephalitis
Virus (EEEV)
Echovirus
Epstein-Barr Virus (EBV)
Hepatitis A Virus (HAV)
Hepatitis B Virus (HBV)
Hepatitis C Virus (HCV)
Hepatitis Delta Virus (HDV)
Hepatitis E Virus (HEV)
Herpes Simplex Virus 1 (HHV1)
Herpes Simplex Virus 2 (HHV2)
Human Immunodeficiency Virus
(HIV)
Human T-lymphotrophic Virus
(HTLV)
Norwalk Virus
Papilloma Virus (HPV)
Polio virus
Rhinovirus
Rubella Virus
Saint Louis Encephalitis Virus
Varicella-Zoster Virus (HHV3)
Western Equine Encephalitis Virus
(WEEV)
Yellow Fever Virus
Icosahedral viruses

15. 3. Complex.
 E.g. poxvirus (the virus
that causes smallpox in
humans)
 Another well known
example is the tailed
bacteriophages such as T4.
 The head of these viruses is
cubic with a triangulation
number of 7. This is
attached by a collar to a
contractile tail with helical
symmetry.
T4 Bacteriophage

16. Shapes of Viruses
Bullet-shaped
Filament
Spherical
Rod-shaped
Brick-shaped
Tadpole-shaped

18. Major Components of Viruses
1. NUCLEIC ACID:
 The genome (set of genes) of virus maybe genes or RNA but not
both of them.
2. VIRAL CAPSID:
 It is a protein shell, or coat, that encloses the nucleic acid
genome.
 Functions:
 Protect the viral nucleic acid.
 Participate in the viral infection.
 Share the antigenicity
3. NUCLEOCAPSID
The core of a virus particle consisting of the genome
plus a complex of proteins.
 Complex of proteins=Structural proteins +Non- Structural proteins
(Enzymes &Nucleic acid binding proteins)

19. 4. ENVELOPE:
 It is a lipid bilayer membrane surrounding the
nucleocapsid(i.e capsid and genome) of some
viruses. The envelope carries glycoproteins, which
form projection or spikes.
 The virus that has envelope is called enveloped virus
but the one that lacks the envelope is called naked
virus.
 Most animal viruses are enveloped unlike most plant
viruses.
 It is acquired during viral maturation by a budding
process through a cellular membrane (except
poxviruses)
 Possibility of exiting cell without killing it
 Contains at least one virally coded protein
 Attachment protein

20. Functions of envelope
Antigenicity
Some viruses possess neuraminidase
Infectivity- Loss of envelope results in loss of
infectivity
Resistance

21. Properties of enveloped viruses
Labile in dry , arid environment
Damaged by drying, acid, detergent, and heat
Pick up new cell membrane during multiplication
Insert new virus-specific proteins after assembly
Virus is released by budding
Consequences of Properties for enveloped viruses
Must stay moist
Must not infect the GI tract for survival
Must be transmitted in the protective, droplets, secretions, blood and
body fluids
Does not need to kill host cell to spread
Must reinfect another host cell to sustain
Humoral, Antibody and cell-mediated immunity are needed to control
the infection
Elicits hypersensitivity and inflammation to cause immunopathogenesis

22. Enveloped viruses
California Encephalitis Virus
Coronavirus
Cytomegalovirus (CMV)
Eastern Equine Encephalitis
Virus (EEEV)
Epstein-Barr Virus (EBV)
Hantavirus
Hepatitis B Virus (HBV)
Hepatitis C Virus (HCV)
Hepatitis Delta Virus (HDV)
Herpes Simplex Virus 1 (HHV1)
Rotavirus
Rubella Virus
Saint Louis Encephalitis Virus
Smallpox Virus (Variola)
Vaccinia Virus
Herpes Simplex Virus 2 (HHV2)
Human Immunodeficiency Virus
(HIV)
Human T-lymphotrophic Virus
(HTLV)
Influenza Virus (Flu Virus)
Molluscum contagiosum
Papilloma Virus (HPV)
Polio virus
Rhinovirus
Varicella-Zoster Virus (HHV3)
Venezuelan Equine Encephal. Vir.
(VEEV)
Western Equine Encephalitis
Virus (WEEV)
Yellow Fever Virus

23. Properties of naked viruses/ Nucleocapsid
Remain stable and infectious in hostile
environment. Not damaged by drying, acid,
detergent, and heat
Released from host cell by lysis
Consequences
Can infect the GI tract and survive the acid and bile
Can be transmitted easily via hands, dust, fomites, etc
Can stay dry and still retain infectivity
Can resist the antimicrobial effect of detergents and sewage
treatment processes.
Neutralizing mucosal and systemic antibodies against the capsid
are needed to control the establishment of infection

24. Naked viruses( Non Enveloped )
Adeno-associated Virus (AAV)
Adenovirus
B19
Coxsackievirus - A
Coxsackievirus - B
Echovirus
Hepatitis A Virus (HAV)
Hepatitis E Virus (HEV)
Norwalk Virus