Viruses:
Edward Jenner first introduced term “virus”
Dmitri Ivanosky in 1890 discovered virus for first time
Virus is latin word derived from venom meaning Poison.
Viruses are non- cellular with an outer capsid made up of proteins and
inner core of nucleic acid (RNA orDNA).
Viruses ranges from 20-200 nm.
Outside the cell, virus is dormant but once it enters into the cell it takes
over cellular resources and starts replication
Modes of Virus Transmission:
Shedding
Droplet contact or respiratory secretions
Sexual Transmission
Fecal-oral Transmission
Vertical Transmission
Iatrogenic Transmission
Vector borne Transmission
Classification of Viruses:
Viruses are classified on the basis of
Morphology
Nucleic acid
Taxonomy
Type of Disease they cause
Viruses on the basis of Morphology:
Helical Viruses:
In which protein capsid appears in coiled pattern.
Example includes Tobacco mosaic virus and Rabies Virus.
Polyhedral Viruses
In which N. A is surrounded by hollow multi-sided capsid
Example includes Adenoviruses and Polio viruses.
Enveloped Viruses
In which helical or polyhedral viruses are enclosed by envelopes
Examples includes Influenza virus and Herpes simplex virus.
Complex Viruses
In which virus has neither helical nor polyhedral instead combination of both
Examples include Bacteriophage and Poxvirus.
Viruses on the basis of Nucleic Acids:
A) DNA Viruses:
In which genetic material is either single or double stranded DNA and
virus usually replicates using a DNA dependent DNA polymerase
Classification of Virus on basis of DNA:
1) Parvoviruses:
These are very small (22nm in diameter), naked icosahedral virus with
single stranded linear DNA.
Replication occur only in actively dividing cells.
Example includes Adeno-associated viruses and B19 viruses
2) Polyomaviruses:
They are small vuruses of about 40nm
Non-enveloped and heat stable
Cubic symmetry with 72 capsomeres
Have double stranded DNA
Produce latent and chronic infections in host
Examples include IC viruses and BK viruses
3) Papillomaviruses:
These are similar to polyomaviruses in many respects, but they have larger
genome.
These have particle size of about 55nm.
These are very host and tissue specific.
4) Adenoviruses:
These are naked icosahedral viruses with 75 nm diameter with double
stranded linear DNA.
They cause phryngitis, upper and lower respiratory tract infections.
5) Hepadnaviruses:
These are double- shelled viruses 45nm in diameter with an icosahedral
capsid covered by an envelope.
The DNA is circular double stranded
They cause acute and chronic hepatitis
6) Herpesviruses:
These are enveloped viruses 100nm in diameter.
Have an icosahedral nucleocapsid and double stranded linear DNA.
They are noted for causing latent infections.
7) Poxviruses:
These are the largest viruses, with bricklike shape and envelope with an
unusual appearance and a complex capsid symmetry.
They are named for skin lesion.
Small pox virus and vaccinia virus are two important members.
B) RNA viruses:
An RNA is a virus that has RNA as its genetic material.
Usually these viruses have single-stranded RNA but may also have double
stranded RNA.
Examples includes SARS, influenza, Heptatitis C, West Nile fever and Polio
Classification of Virus on basis of RNA:
1) Picornaviruses:
These are smallest 28 nm in diameter RNA viruses.
Thet have single stranded liner non segmented positive polarity RNA
within a naked icoshedral capisd.
There are two groups of human pathogens i.e. enterovirus and
rhinovirus.
2) Calciviruses:
These are naked viruses 38 nm with icosahedral capsid having single
stranded liner non segmented positive polarity RNA.
The two group of human pathogens includes Norwalk virus and
hepatitis E virus.
3) Astroviruses:
Similar to size of picno viruses 28-30 nm.
Particles display a distinctive star shaped outline on their
surface.
Their genome is linear, positive sense, single stranded RNA.
4) Reoviruses:
These are naked viruses 75 nm with icosahedral capsid coats.
They have 10 segments of double stranded linear RNA.
The main human pathogens is rotavirus which cause diarrhoea mainly in
infants.
5) Togaviruses:
These are enveloped viruses with an icosahedral capsid and single stranded
linear non segmented positive polarity RNA.
There are two major groups of human pathogens:
Alpha viruses & Rubiviruses
6) Flaviviruses:
These are enveloped viruses 40-60nm with icosahedral capsid and single
stranded linear non segmented, positive polarity RNA.
Their genome size ranges from 9.5 to 12.5 kb
Examples includes HCV, Yellow fever virus, dengue virus, west Nile virus,
St Louis and Japanese virus.
7) Arenaviruses:
These are Polymorphic, enveloped viruses ranging in size from 300-500nm.
The genome is segmented, circular, single stranded RNA, negative-sense and
10-14 kb total size.
8) Retroviruses:
These are enveloped viruses with an icosahedral capsid and two identical
strands of single stranded linear, non segemented, positive polarity RNA.
9) Orthomyxovirus:
These virus are enveloped with a helical nucleocpsid and eight segments of
linear single stranded negative Polarity RNA.
Influenza is the main human pathogen.
10) Paramyxovirus:
These are enveloped viruses with a helical nucleocapsid and single stranded
linear,non segmented, negative-polarity RNA.
The main important human pathogens are measles mumps and Para
influenza.
11) Rhabdoviruses:
These are bullet-shaped enveloped viruses with a helical nucleocapsid and a
linear, single –stranded negative-polarity RNA.
Rabies virus is the only important human pathogen.
12) Bunyaviruses:
These are enveloped viruses with a helical nucleocapsid and a single stranded
circular negative-polarity RNA in three segments.
Hanta viruses such as Sin nombre virus are important member of this family.
13) Delta virus:
Hepatitis delta virus (HDV) is only member of this member of this genus.
It is an enveloped virus with RNA genome that is a single-stranded negativepolarity, covalently closed circle.
It is defective virus because it cannot replicate unless hepatitis B virus (HBV) is
present within same cell.
14) Corona viruses:
These are enveloped viruses with a helical nucleocapsid and a single
stranded liner nonsegemented positive polarity RNA.
Corona viruses causes respiratory tract infections, such as common cold
and SARS (severe acute respiratory syndrome).
15) Filo viruses:
These are enveloped viruses with a helical nucleocpsid and single
stranded,linear,non segmented negative polarity RNA.
They are highly pleomorphic, long filaments that are 80 nm in diameter but
can be of thousand nanometre long.
Two important human pathogens are Ebol virus and Marburg.
Families of viruses effecting Human