Viruses are the smallest infectious agents that can only replicate inside host cells. They contain either DNA or RNA, but lack the cellular machinery to reproduce. Viruses come in a variety of shapes and sizes, with capsids composed of protein subunits that surround the viral genome. They are cultivated by infecting animals, eggs, or cell cultures. Laboratory diagnosis of viral infections involves directly observing viruses, isolating them in culture, or detecting antibodies produced in response to infection. Viruses are classified based on their nucleic acids and other properties.

1. General properties of
viruses

2. VIRUSES
Viruses are the smallest obligate intracellular
infective agents containing only one type of
nucleic acid (DNA or RNA) as their genome.
They do not possess a cellular organization
and lack the enzymes necessary for protein and
nucleic acid synthesis, They are resistant
differences to antibiotics.

3. I. MORPHOLOGY OF VIRUSES.
Size
Viruses are much smaller than other organisms.
The extracellular infectious virus particle is
called the virion. The size of viruses ranges
from 20 to 300 nm in diameter. The largest
virus is the smallpox virus (300 nm) and the
smallest is the parvovirus (20 nm)

5. Structure and Symmetry
1. STRUCTURE
The virion consists of a nucleic acid core
(genome) surrounded by a protein coat, the
capsid. The capsid together with the enclosed
nucleic acid is known as the nucleocapsid. The
capsid is composed of a c large number of
protein subunits (polypeptides) which are
known as capsomers. Certain viruses also
contain e envelope that surrounds the nucleic
acid. It is lipoprotein in nature.

7. SYMMETRY
Three types of symmetry are determined by the
arrangement of capsid around the nucleic acid
core.
(i) Icosahedral (cubical) symmetry
(ii) Helical symmetry
(iii) Complex symmetry
SHAPE
The overall shape of virus particles varies in
different groups . Pox virus is brick-shaped and
rabies virus is bullet shaped.

9. CHEMICAL PROPERTIES
NUCLEIC ACID
Viruses contain only one kind of nucleic acid,
either single or double stranded DNA or RNA.
VIRAL PROTEIN AND LIPIDS
Viruses contain protein which makes up the
capsid. In case of enveloped viruses, they
contain lipids (present in envelope).

10. CULTIVATION OF VIRUSES
As viruses multiply only in living cells, they
cannot be grown on any of the inanimate culture
medium. Three methods are employed for the
cultivation of viruses:
A. Animal inoculation.
B .Embryonated egg inoculation
C. Cell culture Parvovirus

11. • A. ANIMAL INOCULATION
Infant (suckling) mice are used in the isolation
of arboviruses and coxsackie viruses, many of
which do not grow in any other system. After
inoculation, animals are observed for signs of
disease or death. Later on, they are sacrificed
and tissues are tested for the presence of virus.

13. B. Embryonated Egg
Inoculation
• Embryonated hen's eggs
(7 to 12 days old) are
inoculated by one of the
several routes such as
amniotic sac yolk sac
and allantoic cavity

14. C. CELL CULTURE/TISSUE CULTURE
This is the type of culture routinely employed
for diagnostic virology. Cell cultures are
classified into three different types
(i) Primary cell culture
(ii) Diploid cell strains
(iii)Continuous cell lines

16. • (i) Primary cell cultures
These are normal cells freshly taken from the organs of
animal or human being and cultured. They are
capable of very limited growth in culture, perhaps 5-
10 divisions at the most.
(ii) Diploid cell strains
They can be subcultured for a limited number. After
about 50 serial subcultures they undergo senescence'
and the cell strain is lost.

17. (iii) Continuous cell lines
These are cells of a single type that are capable
of indefinite growth in vitro. They are usually
derived from cancerous tissue. They can be
serially cultivated indefinitely, therefore, they
are termed continuous cell lines.

18. LABORATORY DIAGNOSIS
OFVIRAL INFECTIONS
• A. Direct demonstration of virus and its components
• B. Isolation of virus
• C. Detection of the specific antibodies
DIRECT DEMONSTRATION OF VIRUSAND ITS
COMPONENTS
1. Electron Microscopy
The detection of virus by electron microscopy (EM) is
being used increasingly especially for viruses that
are difficult to culture. Clinical applications of
electron microscopy include detection of rotavirus
and hepatitis

19. 2. Fluorescent Microscopy
3. Light Microscopy
Inclusion bodies in tissue sections may be
detected by light microscopy. Demonstration
of inclusion bodies helps in diagnosis of some
viral infections.

20. 4.Viral Antigens
These may be detected by enzyme linked
immunosorbent assay (ELISA) and latex
agglutination.

21. 5. Polymerase Chain Reaction (PCR)
With a PCR technique, a target DNA sequence can
be amplified to the point where it can readily be
identified using labelled probes in a hybridisation
assay. Thus viral DNA extracted from a very small
number of virions or infected cells can be detected.
The technique can be used for the diagnosis of
infections caused by HIV-1, HIV-2, hepatitis B
virus, and Epstein-Barr virus.

22. • C. DETECTION OF SPECIFIC
ANTIBODIES
The demonstration of a rise in titre of antiviral
antibodies during the course of a disease is
strong evidence that it is the aetiological agent.
For this, paired sera should be collected from
the patient, the acute sample collected early in
the course of the disease and the convalescent
sample collected ten to fourteen days later.
Examination of a single sample of serum is
meaningful when IgM specific antibodies are
detected.

23. CLASSIFICATION OF VIRUSES
• Viruses are classified on the basis of physical
and chemical biological, properties. Viruses
are broadly classified into DNA and RNA
viruses

24. PRIONS
Prions are infectious proteins without any
detectable nucleic acid. They are highly
resistant to physical and chemical agents. They
are resistant to heat (90°C for three minutes),
UV rays and nucleases and sensitive to
proteases. They produce slow infections with
long incubation period (in years).