• Throughout the course of modern human history, the source some viral infections such as smallpox, polio, and the Spanish flu have been quite unknown to humans. They have been diseases which have had deadly effects on humanity. All that was known about these diseases was that they spread via person-to-person contact.
  • In the second half of the1800’s, Louis Pasteur postulated that the disease rabies was caused by a “living thing” which is in all probability, smaller than bacteria itself. This postulation of his led him to develop the first vaccine against rabies in 1884. 
  • The initial discovery of the light microscope held some promise with respect to the observation of the agents causing such sever diseases however, it was found that the microscope could only be used to observe bacteria, protozoa and fungi. The size of virus was smaller than these agents and therefore, could not be observed under a microscope.
  • In the 1890’s, D. Ivanovski and M. Beijerinck showed that a disease caused in plants was cause by the tobacco mosaic virus which served as the first properly substantial revelation related to viruses.
  • This discovery was then followed by two other scientists, Friedrich Loeffler and Paul Frosch who isolated the virus that caused foot and mouth disease in cattle. 


Viruses are extremely unique entities which are capable of infecting almost every single type of cell known including bacteria, algae, fungi, plants, animals and even protozoa. 

Questions regarding the nature of viruses like their origination, their state of existence (alive or non-living), their distinct biological characteristics etc. are still very dominant in the scientific world.

Some ideas that have been addressed are:

  • Viruses are considered to be the most abundant microbes on earth, in terms of number.
  • Viruses are considered to be obligate intracellular parasites that are incapable of dividing or multiplying unless they invade a specific host cell. They multiply by taking over the hosts genetic and metabolic machinery of the cells of the host.
  • They are said to have been in existence for billions of years and have arisen from the loose strands of genetic materials released by the cells. (This is one widely accepted theory. However, it too has faced some criticism.)


  1. Viruses are obligate parasites of protozoa, bacteria, fungi, animals, plants, and algae.
  2. They have an ultramicroscopic size range from 20nm up to 450 nm in diameter.
  3. They have a very compact and economical structure. They are not cellular in nature.
  4. They are inactive macromolecules outside the host cell and only get activated inside the host cells.
  5. Viral nucleic acid can be DNA or RNA but never both together.
  6. Their basic structure consists of a protein coat (the capsid) surrounding the nucleic acid core.
  7. They lack the enzyme and machinery for basic metabolic processes and synthesis of proteins.
  8. Virus multiply by taking over the host machinery and genetic material.



  • Viruses represent the smallest infectious agents in the biological world. (with a few exceptions)
  • They lie within the ultramicroscopic size range with sizes usually less than 0.2 micrometre. They are so small that one requires an electron microscope to detect or examine their structure.
  • The animal size range may vary from parvoviruses (which are around 20 nm in diameter) to megaviruses and pandoraviruses (which are up to 1000nm in width) that may be as big as small bacteria.
  • Some viruses which are cylindrical may be relatively long (around 800nm) but have a very narrow diameter (around 15nm).
  • Negative staining using an opaque salt in combination with electron microscopy can be used for the observational studies of viruses.


Viruses have a crystalline appearance due the occurrence of regular, repeating molecules. A number of purified viruses even form large aggregates and crystals when subjected to special treatments. 

The general plan of all viruses or the general architecture is quite simple. Almost all viruses contain a protein coat or the capsid which encloses the viral genome which may be a DNA or an RNA sequence. Apart from this, viruses only contain those parts which are needed to invade and take control over the hosts cellular machinery. 

Some important terms are:

  • Capsid: The outer covering or the shell of the virus that surround the central nucleic acid core of the virus.
  • Nucleocapsid: The outer shell along with the nucleic acid core is called the nucleocapsid.
  • Naked viruses: Viruses that do not contain a nucleocapsid layer are called naked viruses.
  • Enveloped viruses: Some virus classes possess an additional covering which is external to the capsid which is called an envelope. This envelope structure is usually a piece of the hosts cell membrane. These types of viruses are called enveloped viruses.


The capsid layer of the virus, when magnified immensely, shows the appearance of small, prominent, geographic structures. These structural subunits of the capsid are called capsomeres. 

These capsomeres are capable of self-assembling into the finished capsid structure. Depending on the shape and assembly of the capsomeres, the resulting structure can be of two types; 

  1. Helical: Helical capsids have rod shaped capsomeres that bind together to form a structure similar to hollow discs (like a bracelet). During the process of formation, these discs link together to form a continuous helix.See the source image
  1. Icosahedral capsids: An icosahedron is a three-dimensional, 20-sided figure with 12 evenly spaced corners. Some viruses also show such an arrangement in this shape.
  2. Complex viruses: Such viruses may have a specific head, a neck and other structures specific for the invasion of host cells. The most common examples of such viruses is phage viruses.


Enveloped viruses, when released from the host cells sometimes carry forward a piece of the hosts cell membrane with them in the form of an envelope. Although it is derived from the host, the envelope is different in the virus because the normal proteins of the host get replaced with the viral proteins. 


At the center of the viral structure, within the capsid lies the viral genome which may be single or double stranded. The genetic material may be RNA or it may be DNA but it is never both even in viruses. The genome may be a few hundred to thousands of base pairs long. 


Foundations in Microbiology – Talaro, Kathleen P


BY: ABHISHEKA G (MSIWM013)          


HIV (Human immunodeficiency virus) is a kind of virus that belongs to the retrovirus family and causes AIDS (Acquired immunodeficiency syndrome) in humans. It destroys the immune system of the individual by making the patient vulnerable to different other infections including neurological disorders. The disease of AIDS was first observed in 1981 in the united states. It is an epidemic disease present throughout the world. This disease can be preventable, but not curable.

TYPES OF HIV: There are two types of HIV viruses are present based on the origin of the virus.

  1. HIV-1: It is isolated in America, Europe, and Central Africa. It is more virulent and spread among individuals very rapidly
  2. HIV-2: It is isolated in West Africa, it is less virulent and does not spread rapidly as HIV-1


  1. It is spherical in nature and measures about 90-120nm in diameter and it is composed of two copies of positive single-stranded RNA enclosed by a conical capsid composed of viral protein P24. The capsid also contains the enzymes which are necessary for viral replication such as Reverse transcriptase (P55/66), Integrase(p32), and protease(p10).
  2. The RNA genome of HIV consists of nine different genes that contain the information needed to make the structural proteins for new virus particles.
  3.  The matrix is composed of the viral protein P17 which surrounds the capsid ensuring the integrity of the virion particles. And the matrix is surrounded by 2 layers of phospholipids which are embedded by 70 copies of viral glycoprotein and lipoprotein which consist of two units namely anchoring transmembrane pedicles (Gp41) and surface projecting knob like spikes (Gp120).

MODE OF TRANSMISSION OF HIV VIRUS:  HIV is transmitted in many different ways.

1.Sexual contact: Unprotected sexual relationship with an infected individual.

2. From mother to child: The virus of HIV is transmitted from mother to child through the uterus, during delivery, and also during breastmilk.

3. Through needles: Through the injections during injecting drugs and needle prick injury which are already used by the infected individuals.

4. Blood transfusion and organ transplantation: The virus is transmitted through transfusion of blood and transplantation of organs from infected individuals to normal individuals.


There are 7 different steps in the life cycle of the HIV virus.

  1. Attachment to the host/Binding to the target cell: The specific site for the HIV virus to bind are CD4 receptors and other co-receptors(CCR5/CXCR4) which are enveloped by the glycoprotein Gp120. Viral binding to host cell triggers fusion of the viral and cell membrane of the host cell, which is mediated by gp41 and allows the virus core into the cytoplasm of the host cell.
  2.  Reverse transcription of its RNA: HIV consists of single-stranded RNA. The single-stranded RNA is converted into DNA. This process is carried out by the enzyme Reverse transcriptase.
  3. Integration into the host genome: The viral double-stranded DNA is integrated into the genome of the infected host cell through the action of the viral integrase enzyme which causing a latent infection.
  4. Transcription of viral proteins: The transcription of viral proteins is mediated by Viral reverse transcriptase. The infected DNA of the host cell makes proviral RNA and protease enzyme cuts or cleaves the polypeptides into functional HIV proteins and the virion assembles.
  5. Assembly of a new viral particle: Viral strands and other proviral products combine to form a package and head for the cell membrane.
  6. Release of immature virions: Immature virus finds a suitable position to push or come out of the host cell by taking a piece of the membrane with it.
  7. Maturation of the virion: The new free virus matures and gets ready to infect another host cell.


Diarrhea, Fatigue or weakness, fever, headache, joint pain, night sweats, rashes on the body, swollen glands, sudden weight loss, yeast infections in the body, and sexual organs that occur frequently and last for a long time.

LABORATORY DIAGNOSIS OF HIV: HIV is mainly detected by  different clinical tests:

  1. ELISA (Enzyme-linked immunosorbent Assay/ Enzyme Immunoassay): This test detects and measures the antibodies in the blood sample along with the presence of antibodies related to any infections.
  2. Western Blot: This test is used to detect the specific proteins called HIV antibodies present in the blood sample. If ELISA gives positive for HIV, then the western blot technique is used to confirm the positive result of ELISA and gives 99.9% accurate results.


  1. Nucleoside reverse transcriptase: Zidovudine and Stavudine
  2. Non-nucleoside reverse transcriptase inhibitor: Efavirenz and Nevirapine.
  3. Protease inhibitor: Atazanavir and Darunavir.


  1. Avoid multiple sexual contacts.
  2. Using new and sterile needles for injection.
  3. Care should be taken and testing should be done during blood transfusion and pregnancy.
  4. Biomedical waste should be properly disposed of from hospitals and households.
  5.  Proper sex education should be provided to people.






Virus are small structure which can pass its genetic material to living host where it can replicate. They can infect plants, animals and other microorganisms also. The study of viruses is called virology which is considered as a subspecialty of microbiology.  Martinus Beijerinck is the father of virology.

In 1898, Friedrich Loeffer and Paul Frosch was the first to find evidence of the nature of viruses, genetic entities of virus. They found the evidence from foot and mouth disease in livestock. The first identified human virus was yellow fever caused by mosquito Aedes aegyptii.Virus was discovered and reported in 1910 by a US Army physician.Veterniary virology gained importance after in1902 after cattle plague.


Virus classification is the process of naming virus and places them into a taxonomic system. Virus are mainly classified by phenotypic characters like morphology, mode of replication, host organisms, nucleic acid type and the type of disease caused by the virus. A universal system for classifying viruses, and a unified taxonomy was established in1966 by International Committee on Taxonomy of Viruses (ICTV).vanRegenmortel (19) lists the following characters for discriminating between virus species:

  • Relatedness of genome sequence
  • Natural host range
  • Cell and tissue tropism
  • Pathogenicity and cytopathology
  • Mode of transmission
  • Physicochemical properties of viral proteins
  • Antigenic properties of viral proteins.

The system makes use of a series of ranked taxons,with the: order,family,subfamily,genus,species,order,family,genus,species.


It is classified based on shape and size. Based on the structure of nucleocapsid it’s divided into two namely helical morphology and icosahedral morphology. Helical morphology consist of a helical array of capsid proteins which is surrounded by a helical filament of nucleic acid. The number and arrangement of the capsomers are useful in identification and classification of icosahedral morphology Based on envelope it’s divided as enveloped and non-enveloped virus.


  • It possess no membranes,cytoplasm,ribosomes or other cellular components
  • They cannot move or grow
  • They are really tiny, even smaller than cell and is only visible advanced electron microscopes.
  • It consist of two major parts which are a protein coat and a hereditary material which can be a RNA or DNA.


Viruses cannot replicate on its own and in order get multiplied it must infect a host. It replicates in host cell’s machinery to create more viruse.steps involved in replication includes:

  • Penetration: The virus gets engulfed by the cell or get attached to some other viruses which have surface proteins that bind to receptors on the host cell. It allows the host cell to either fuse with virus or absorb the virus. Once it reaches the cell, the genetic material is released to the cytoplasm.
  • Replication: The genetic material of the virus is copied several times.
  • Transcription: The genetic material act as a blue print for the cell to make messenger RNA which is used to make viral proteins.
  • Protein Assembly: It occurs in the cytoplasm (ribosome) where viral proteins are made.
  • Viral Assembly: The viral genetic material (after replication) will get surrounded by the newly made viral protein.
  • Release: Through budding viruses get emerged from the cell through cell membrane or by bursting out of the cell which will result in the death of host cell.


Size of a virus is smaller than bacteria and size is variable. The larger viruses can vary their size about 300 A in diameters. It indicates that it may be as larger as a small bacteria. They occur mainly in three shapes which are helical (spherical or can be of complex symmetry. Virus lacks cytoplasm and hence lack cell organelles like mitochondria, Golgi bodies, ribosomes and enzyme systems are also present. Virus contains RNA or DNA whereas a normal cell contains both DNA and RNA. It contains a protein coating called capsid which acts as a protection for genetic materials by surrounding the genetic material. A virus particle is called virion.

Plant virus contains only RNA.Animal virus contains both RNA or DNA.wheras bacteriophages contain only DNA.


  • Double-Stranded DNA: Double-Stranded DNA virus generally have a polyhedral or complex structure. Examples are Papilloma Virus,Variola(smallpox)
  • Double-Stranded RNA: Double-Stranded RNA usually have polyhedral structure. Example is diarrhea virus.
  • Single –Stranded RNA: Single –Stranded RNA virus have two subunits which can serve as mRNA and other one which can serve as a template mRNA.Examples are Rhinovirus,HIV


The tumor viruses change cells by integrating their genetic material with the host cell DNA.This is a permanent insertion in the genetic material which is never removed. The insertion mechanism can differ depending on whether the genetic material of the tumor causing DNA or RNA.In case if the genetic material is DNA then it can be directly inserted into the host DNA.In case of RNA as the genetic material, then RNA is first transcribed into DNA and then it is inserted into the host cell’s DNA.



Structure, Characteristics and classification


  • Virus is a parasite of sub-microscopic level on all the organisms. They infect all type of life forms like plants, animals, bacteria.
  • They are ambiguous in nature i.e. weather living or non-living, they are non-living in free-state but behave like living organism in a host.
  • In 1892  Dmitri Ivanovsky’s described about virus when he was working on bacterial pathogen infecting tobacco plants.
  • When virus invades a host organism it forced to replicate rapidly to produce thousands of copies of the virus. In contrast when virus is not inside a host or in the process of infecting it, virus exists in independent form known as ‘virions’ consists of a genetic material (RNA or DNA), a protein coat – capsid and the outer envelope of lipids.
  • Virus can be of various shapes ranging from helical to the icosahedral form.
  • Virus are much smaller than bacteria, size of the most virus that have been studied is between 20-300 nm.
  • They contain only one type of nucleic acid at a time either DNA or RNA but never both.
  • Virus are responsible for various infectious diseases in plant and animals rabies, AIDS (HIV), avian influenza, Ebola virus disease. Transmission of viral disease is through ‘vector’ (diseases bearing organism).


Virus shows a wide variety of shape and size, a complete virus particle know as virions contain nucleic acids surrounded by the protein coat capsid, capsid is made up of small subunits of protein knows as capsomers and an outer layer called envelope made up of lipid which is derived from host cell membrane.

The main morphological structures are:


  • Composed of single type of capsomeres stacked around the central axis of helical structure having central cavity results in the formation of rod shape structure which may be short and rigid or long and flexible
  • Genetic material (typically ssRNA or ssDNA) bound to the protein helix by the interaction between the negatively charged nucleic acid and the positively charged capsomer protein.

Example: tobacco mosaic virus


  • Most animal viruses exhibit icosahedral symmetry. The minimum no of triangular faces is three which gives rise to the 60 more capsomers. Rotavirus has more than 60 capsomers. Regular icosahedron is the optimum way to form closed shell symmetry with identical subunits.
  • Capsomers attaches with 5 other capsomers at the apices called pentons and on the triangular faces attaches with 6 capsomers called hexon.
  • Pentons and hexon may be of same protein or may be of other proteins.


  • Elongation of icosahedron along the fivefold axis, it is common in bacteriophage and composed of a cap with cylinder structure.


  • In some species, the virus modify their cell membranes to envelope them self it may be either the outer membrane of the host or the internal nuclear membrane or endoplasmic reticulum making a outer lipid bilayer known as viral envelope.
  • The infectivity are depend on the envelope of the virus, the membrane i
  • Has protein coded by the viral and host genome and lipid membrane with any carbohydrates originate by the host.


  • Non- cellular organism enclosed by a protective envelope
  • Virus attaches to their host by the help of spikes
  • Having nucleic acid (DNA and RNA) in the core which is surrounded by the protein coat
  • Considered as both living and non-living i.e. inactive when present outside the host and became active within the host cells
  • Virus uses host mechanism and enzymes to reproduce itself.

Classification :

On the basis of genetic material:

DNA virus:

  • DNA as genetic material
  • They attack on both humans and animals
  • Example: papillomavirus, parovirus and herpesvirus

RNA virus:

  • RNA as genetic material
  • Example: polio virus, ebola virus, hepatitis C virus

DNA-RNA viruses:

  • Having both DNA and RNA as genetic material
  • Example: leukoviruses, rous’s viruses

On the presence of number of strands:

Double-stranded DNAherpes viruses, adenoviruses
Single-stranded DNAbacteriophagesφ, X, 74 bacteriophages
Single-stranded RNAinfluenza virus, poliomyelitis, bacteriophage MS-2
Double-stranded RNArice dwarf viruses, wound tumour virus

On the basis of envelope:


DNA viruses:  poxviruses, herpesviruses

RNA viruses: toga virus, coronavirus


DNA viruses: adenovirus, papovirus

RNA viruses: hepatitis A and E virus

On the basis of capsid structure:

Naked icosahedralPoliovirus, hepatitis A virus
Enveloped icosahedralRubella virus, HIV-1
Naked helicalTMV
Enveloped helicalMumps virus, measles virus
ComplexSmallpox virus, hepatitis B virus

On the basis of shape:

Space vehicle shapedAdenovirus
Filamentous shapedEbola virus
Brick shapedPoxvirus
Bullet shapedRabies virus

On the basis of type of host:

Animal viruses:

  • Viruses infect and live inside the animal cell
  • Example: rabies virus, mumps virus

Plant viruses:

  • Their genetic material is RNA remains enclosed in the protein coat and infect plants
  • Example: potato virus, TMV, turnip yellow viruses


  • Virus infect bacteria are known as bacteriophage, contain DNA as genetic material

On the basis of mode of transmission:

Transmitted throughExample
Respiratory routeRhino virus, swine flu
Faeco-oral routePolio virus, rota virus
Blood transfusionHIV, hepatitis B virus
Sexual contactRetro virus
Zoonotic virusAlpha virus, flavi virus

Baltimore Classification:

  • In early 1970s Nobel prize winner David Baltimore developed the most commonly used virus classification system
  • Baltimore focus on how mRNA is produced during replication and classify virus into various groups

Group I: mRNA is produced as the same way as their cellular DNA by transcription, contain ssDNA as their genetic material

Group II: convert their ss genome dsDNA before transcription of mRNA occur, have ssDNA as genetic material

Group III: uses RNA dependent RNA polymerase to generate mRNA from the one of the strand used as template, genome is dsRNA.

Group IV: genetic material is ssRNA . Genomic RNA is with positive polarity means that is directly serve as mRNA

Multiple full length RNA strand with negative polarity are formed from the intermediate of dsRNA (replicative intermediates) made in the genomic copying process.

These serve as template form the production of positive polarity RNA

Group V: contain ssRNA with negative polarity; means that sequence is complementary to mRNA. Negative-strand is converted into mRna

Group VI: virus have two copies of genome i.e. ssRNA  conveted using reverse transcriptase enzyme to dsDNA

dsDNA transported to the host’s nucleus in inserted in its genome viral DNA is produced by transcription with host DNA

group VII: genetic material is dsDNA which make ssRNA as intermediate acts as mRNA and also converted back to dsDNA  by reverse transcriptase enzyme.