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Introduction to viruses

This topic explains about the History of virus discovery, Structure and morphology of viruses ,General characteristics of viruses ,Pathogenicity and epidemiology of viral infections

Classification of viruses:

This topic explains about the different modes of classification of viruses like , Based on envelope- Enveloped and non-envelope; based on genetic material-DNA,RNA, single and double stranded; based on mode of infection- air borne, water borne, STD, zoonoses

Viral genomic organization and replication

Viral Structure and Replication

Viruses are noncellular genetic elements that use a living cell for their replication and have an extracellular state. Viruses are ultramicroscopic particles containing nucleic acid surrounded by protein, and in some cases, other macromolecular components such as a membranelike envelope.

Outside the host cell, the virus particle is also known as a virion. The virion is metabolically inert and does not grow or carry on respiratory or biosynthetic functions.

At present, there are no technical names for viruses. International committees have recommended genus and family names for certain viruses, but the process is still in a developmental stage.

Viruses vary considerably in size and shape. The smallest viruses are about 0.02 μm (20 nanometers), while the large viruses measure about 0.3 μm (300 nanometers). Smallpox viruses are among the largest viruses; polio viruses are among the smallest.

Viral structure. Certain viruses contain ribonucleic acid (RNA), while other viruses have deoxyribonucleic acid (DNA). The nucleic acid portion of the viruses is known as thegenome. The nucleic acid may be single-stranded or double-stranded; it may be linear or a closed loop; it may be continuous or occur in segments.

The genome of the virus is surrounded by a protein coat known as a capsid, which is formed from a number of individual protein molecules called capsomeres. Capsomeres are arranged in a precise and highly repetitive pattern around the nucleic acid. A single type of capsomere or several chemically distinct types may make up the capsid. The combination of genome and capsid is called the viral nucleocapsid.

A number of kinds of viruses contain envelopes. An envelope is a membranelike structure that encloses the nucleocapsid and is obtained from a host cell during the replication process. The envelope contains viral-specified proteins that make it unique. Among the envelope viruses are those of herpes simplex, chickenpox, and infectious mononucleosis.

The nucleocapsids of viruses are constructed according to certain symmetrical patterns. The virus that causes tobacco mosaic disease, for example, has helical symmetry. In this case, the nucleocapsid is wound like a tightly coiled spiral. The rabies virus also has helical symmetry. Other viruses take the shape of an icosahedron, and they are said to have icosahedral symmetry. In an icosahedron, the capsid is composed of 20 faces, each shaped as an equilateral triangle (Figure 1 ). Among the icosahedral viruses are those that cause yellow fever, polio, and head colds. 

Figure 1

An array of viruses. (a) The helical virus of rabies. (b) The segmented helical virus of influenza. (c) A bacteriophage with an icosahedral head and helical tail. (d) An enveloped icosahedral herpes simplex virus. (e) The unenveloped polio virus. (f) The icosahedral human immunodeficiency virus with spikes on its envelope.

The envelope of certain viruses is a lipid bilayer containing glycoproteins embedded in the lipid. The envelope gives a somewhat circular appearance to the virus and does not contribute to the symmetry of the nucleocapsid. Projections from the envelope are known as spikes. The spikes sometimes contain essential elements for attachment of the virus to the host cell. The virus of AIDS, the human immunodeficiency virus, uses its spikes for this purpose.

Bacteriophages are viruses that multiply within bacteria. These viruses are among the more complex viruses. They often have icosahedral heads and helical tails. The virus that attacks and replicates in Escherichia coli has 20 different proteins in its helical tail and a set of numerous fibers and “pins.” Bacteriophages contain DNA and are important tools for viral research.

Viral replication. During the process of viral replication, a virus induces a living host cell to synthesize the essential components for the synthesis of new viral particles. The particles are then assembled into the correct structure, and the newly formed virions escape from the cell to infect other cells.

The first step in the replication process is attachment. In this step, the virus adsorbs to a susceptible host cell. High specificity exists between virus and cell, and the envelope spikes may unite with cell surface receptors. Receptors may exist on bacterial pili or flagella or on the host cell membrane.

The next step is penetration of the virus or the viral genome into the cell. This step may occur by phagocytosis; or the envelope of the virus may blend with the cell membrane; or the virus may “inject” its genome into the host cell. The latter situation occurs with the bacteriophage when the tail of the phage unites with the bacterial cell wall and enzymes open a hole in the wall. The DNA of the phage penetrates through this hole.

The replication steps of the process occur next. The protein capsid is stripped away from the genome, and the genome is freed in the cell cytoplasm. If the genome consists of RNA, the genome acts as a messenger RNA molecule and provides the genetic codes for the synthesis of enzymes. The enzymes are used for the synthesis of viral genomes and capsomeres and the assembly of these components into new viruses. If the viral genome consists of DNA, it provides the genetic code for the synthesis of messenger RNA molecules, and the process proceeds.

In some cases, such as in HIV infection (as discussed below), the RNA of the virus serves as a template for the synthesis of a DNA molecule. The enzyme reverse transcriptase catalyzes the DNA's production. The DNA molecule then remains as part of the host cell's chromosome for an unspecified period. From this location, it encodes messenger RNA molecules for the synthesis of enzymes and viral components.

2 ). 

Figure 2

A generalized representation of the replication of two viruses. Replication of a DNA virus is shown in (1); replication of an RNA virus is displayed in (2).

For the release of new viral particles, any of a number of processes may occur. For example, the host cell may be “biochemically exhausted,” and it may disintegrate, thereby releasing the virions. For enveloped viruses, the nucleocapsids move toward the membrane of the host cell, where they force themselves through that membrane in a process called budding. During budding, a portion of cell membrane pinches off and surrounds the nucleocapsid as an envelope. The replication process in which the host cell experiences death is called the lytic cycle of reproduction. The viruses so produced are free to infect and replicate in other host cells in the area.

Lysogeny. Not all viruses multiply by the lytic cycle of reproduction. Certain viruses remain active within their host cells for a long period without replicating. This cycle is called the lysogenic cycle. The viruses are called temperate viruses, or proviruses, because they do not bring death to the host cell immediately.

In lysogeny, the temperate virus exists in a latent form within the host cell and is usually integrated into the chromosome. Bacteriophages that remain latent within their bacterial host cell are called prophages. This process is a key element in the recombination process known as transduction.

An example of lysogeny occurs in HIV infection. In this case, the human immunodeficiency virus remains latent within the host T-lymphocyte. An individual whose infection is at this stage will not experience the symptoms of AIDS until a later date.

Pox viruses

The pox viruses include the following genera which are important for humans:

o   Orthopox virus: smallpox, alastrim, vaccinia, cowpox, monkeypox, camelpox, buffalopox

o   Parapox virus: orf, pseudocowpox (syn. pseudovaccinia or milker's nodules), sealpox, bovine papular stomatitis, deer pox

o   Molluscipox virus: molluscum contagiosum

o   Yatapox virus: tanapox, yabapox

Herpes viruses

This topic covers, There are 8 herpesviruses which are known to infect humans:

Ø Herpes Simplex virus 1 and 2

Ø Varicella-Zoster virus

Ø Cytomegalovirus

Ø Epstein-Barr virus

Ø Human Herpesvirus 6 (and 7)

Ø Human Herpesvirus 8

Ebola viruses

Ebola disease—also called Ebola hemorrhagic fever or Ebola fever—is a rare and often fatal illness that humans and nonhuman primates (such as monkeys and gorillas) can contract. There have been several outbreaks of Ebola fever in Africa. 


Parainfluenza, mumps, measles, NDV and simian virus 5 

Hepatitis viruses

This topic covers hepatitis viruses like HAV, HBV, HCV,HDV,HEV

Rabies viruses

Hydrophobia disease


Explains about human adenoviruses which are divided into 6 sub-genera A-F.

Human immunodeficiency virus

This topic explains A-Z in detail about HIV

Prions and virions
o The word derives from ‘proteinaceous infectious particle’; meaning that the infectious agent consists only of protein with no nucleic acid genome.
o Prions are the only known example of infectious pathogens that are devoid of nucleic acid.
Lab diagnosis of viral infections:

There are four approaches to confirming a viral infection in the laboratory. Namely:

  • serology: demonstrating an antibody response in a patient's serum
  • direct detection of viral antigens in a clinical sample
  • virus culture
  • viral nucleic acid detection
Cultivation of viruses:

discusses about egg inoculation, tissue culture, animal inoculation methods


Bacteriophages are viruses that parasitize bacteria. They are commonly referred as “phage”. 

Viral Vaccines:

Discusses about the different types of viral vaccines in human use

Emerging Viral diseases

·        Myxoma virus (Rabbitpox)

·        Influenza and Coronaviruses are constantly emerging 

Antiviral drugs
Viruses are obligate intracellular parasites. They lack both a cell wall and a cell
membrane, and they do not carry out metabolic processes. Viral reproduction uses much
of the host’s metabolic machinery, and few drugs are selective enough to prevent viral
replication without injury to the host.
MLAB-486  -  Clinical Virology
Units Hours
Theoretical Practical
1 0 0
Course Catalog Description
Course Requirements
Grading Policy
Evaluation Type Weight
   1 .  First Mid term Exam 15 % ()
   2 .  Second Mid term Exam 15 % ()
   3 .  Participation 10 % ()
   4 .  Final Theory Exam 60 % ()
Learning Outcome
Week Description - Reading  

Introduction to viruses

  • History of virus discovery
  • Structure and morphology of viruses
  • General characteristics of viruses
  • Pathogenicity and epidemiology of viral infections

Classification of viruses:

  • Based on envelope- Enveloped and non-enveloped
  • Based on genetic material-DNA,RNA, single and double stranded
  • Based on mode of infection- air borne, water borne, STD, zoonoses

Viral genomic organization and replication:

  • Viral genetics
  • Protein- nucleic acid interactions
  • Replication of DNA,RNA, single, double stranded viruses
  • Retroviral replication

Viral genomic organization and replication

  • Pox viruses
  • Herpes viruses
  • Ebola viruses

Viral genomic organization and replication

  • Influenza viruses
  • Paramyxoviridae
  • Hepatitis viruses

Viral genomic organization and replication

  • Enteroviruses- Polio, Rota and other enteric viruses

Rabies viruses


Viral genomic organization and replication:

  • Human immunodeficiency virus

              Oncogenic viruses


Lab diagnosis of viral infections

  • Sampling techniques
  • Processing of samples – Enrichment and concentration
  • Direct methods of detection – light ,electron  and fluorescence microscopy
  • Immnuodiagnosis
  • Nucleic acid based diagnosis- Hybridization, PCR, microarray and sequencing

Cultivation of viruses: 

  • In vitro: embryonated eggs
  • In vivo: experimental animals
  • Ex vivo / In vitro : cell cultures - primary and secondary cell lines Suspension cell cultures and monolayer cell cultures
  • Plants and plant cell cultures


  • T (odd and even), Lambda and M13 phages
  • Their structure, types and uses in Microbiology
  • Typing and application in bacterial genetics

Prevention and control of viral infections:

  • Conventional vaccines – killed and attenuated
  • Modern vaccines – DNA, recombinant DNA/protein vaccines , subunits, peptide vaccines, anti-idiotype vaccines
  • Edible vaccines, immunomodulators

Emerging Viral diseases


Upcoming viral diseases

  • Antiviral drugs


Therapeutic viral drugs