Vineet Singh
Virology
Vineet Singh
1|Página
, Vineet Singh
2|Página
, Vineet Singh
Unit 1: Nature and multiplication of viruses
▪ Viruses are biological entities that cannot replicate independently of a living cell (host cell).
▪ A virus is an obligate intracellular parasite that uses active host cellular machinery to multiply
(viruses do not divide).
▪ Viruses are strict parasites of ATP and protein synthesis machinery.
▪ They have an extracellular form called viral particle or virion that facilitate transmission from one
host cell to another.
▪ To multiply, viruses enter an active cell in which they can replicate, a process called infection.
▪ All viruses make mRNA that are translated by host ribosomes: they are parasites of the host
protein synthesis machinery as they themselves can’t synthesize ribosomes.
▪ Virus particles are produced from the assembly of preformed components whereas other agents
grow from an increase in the integrated sum of their components and reproduce by division.
▪ Virus particles themselves do not grow or undergo division.
▪ Resistant to antibiotics.
Bacteria Virus
May cause infectious diseases Always cause infectious diseases
Living organisms Strict parasites
Metabolically active Metabolically inactive
Sensitive to antibiotics Resistant to antibiotics
Reproduction by binary fission Multiply by assembling viral components
▪ Pluricellular organisms grow and reproduce, microorganisms divide meanwhile viruses
multiply.
▪ Transmission: Viruses persist as far as a stable chain of infection is maintained, spreading from
cell-to-cell in a host or from one host to another.
▪ Two general patterns of viral transmission exist:
✓ Involving one species: i.e Hepatitis A virus, that only infects humans, which are it’s reservoir
✓ Alternate infection between species: Dengue virus, non-human primates are the reservoir.
The main diversity of the virus is found in non-human primates→ they have a stable cycle.
Mosquitos (vectors) serve as the intermediate between hosts, but it can sometimes enter in
contact in humans (zoonosis). Dengue virus has four serotypes (1st serotype isn’t harmful but
the 2nd serotype is life threating).
✓ Viral diseases shared by humans and other animals are called zoonoses → hardly effective
process, but still viable, the virus gets a stable cycle inside a new host. I.e In coronavirus bats
are the reservoir but the COV-19 can now also infect humans.
1. Viruses are everywhere
✓ Viruses are obligate intracellular parasites that are on or in every material on earth.
✓ They can be found in soil, water and air.
✓ They can infect any living being.
✓ Viruses in numbers: 1030-1032 viruses on biosphere (virosphere)
➢ 1030x10-15g (weight of an average virus) =1012 kg biomass
• Average human mass 70 kg x 7x109 = 4.9 x 1011 → Viruses exceed biomass of humans by
2-fold
➢ 10 x 125 nm (height of an average virus) =1.25 x 1020 km→1,3 x 107 light years.
30
• In comparison the distance from earth to the Sun is 0.000016 light years.
✓ Viruses are found in hosts in health and disease.
✓ Humans carry different viruses in latent infections (I.e Human Herpesvirus) → the virus remains
inside the host in an inactive state and are stable inside (non-infectious and non-replicative).
✓ Some viral genomes have become trapped as part of host genome (integration through evolution
of harmless or advantageous viral DNA in the host genome).
3|Página
Virology
Vineet Singh
1|Página
, Vineet Singh
2|Página
, Vineet Singh
Unit 1: Nature and multiplication of viruses
▪ Viruses are biological entities that cannot replicate independently of a living cell (host cell).
▪ A virus is an obligate intracellular parasite that uses active host cellular machinery to multiply
(viruses do not divide).
▪ Viruses are strict parasites of ATP and protein synthesis machinery.
▪ They have an extracellular form called viral particle or virion that facilitate transmission from one
host cell to another.
▪ To multiply, viruses enter an active cell in which they can replicate, a process called infection.
▪ All viruses make mRNA that are translated by host ribosomes: they are parasites of the host
protein synthesis machinery as they themselves can’t synthesize ribosomes.
▪ Virus particles are produced from the assembly of preformed components whereas other agents
grow from an increase in the integrated sum of their components and reproduce by division.
▪ Virus particles themselves do not grow or undergo division.
▪ Resistant to antibiotics.
Bacteria Virus
May cause infectious diseases Always cause infectious diseases
Living organisms Strict parasites
Metabolically active Metabolically inactive
Sensitive to antibiotics Resistant to antibiotics
Reproduction by binary fission Multiply by assembling viral components
▪ Pluricellular organisms grow and reproduce, microorganisms divide meanwhile viruses
multiply.
▪ Transmission: Viruses persist as far as a stable chain of infection is maintained, spreading from
cell-to-cell in a host or from one host to another.
▪ Two general patterns of viral transmission exist:
✓ Involving one species: i.e Hepatitis A virus, that only infects humans, which are it’s reservoir
✓ Alternate infection between species: Dengue virus, non-human primates are the reservoir.
The main diversity of the virus is found in non-human primates→ they have a stable cycle.
Mosquitos (vectors) serve as the intermediate between hosts, but it can sometimes enter in
contact in humans (zoonosis). Dengue virus has four serotypes (1st serotype isn’t harmful but
the 2nd serotype is life threating).
✓ Viral diseases shared by humans and other animals are called zoonoses → hardly effective
process, but still viable, the virus gets a stable cycle inside a new host. I.e In coronavirus bats
are the reservoir but the COV-19 can now also infect humans.
1. Viruses are everywhere
✓ Viruses are obligate intracellular parasites that are on or in every material on earth.
✓ They can be found in soil, water and air.
✓ They can infect any living being.
✓ Viruses in numbers: 1030-1032 viruses on biosphere (virosphere)
➢ 1030x10-15g (weight of an average virus) =1012 kg biomass
• Average human mass 70 kg x 7x109 = 4.9 x 1011 → Viruses exceed biomass of humans by
2-fold
➢ 10 x 125 nm (height of an average virus) =1.25 x 1020 km→1,3 x 107 light years.
30
• In comparison the distance from earth to the Sun is 0.000016 light years.
✓ Viruses are found in hosts in health and disease.
✓ Humans carry different viruses in latent infections (I.e Human Herpesvirus) → the virus remains
inside the host in an inactive state and are stable inside (non-infectious and non-replicative).
✓ Some viral genomes have become trapped as part of host genome (integration through evolution
of harmless or advantageous viral DNA in the host genome).
3|Página
