Chapter 1 Introduction

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Chapter 1 - Introduction

• The impact of viruses on our view of life

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Living or Dead?

• In the strict definition used by virologists, a
  virus is classified as an obligate intracellular
  parasite
• It is dependent on the metabolic and genetic
  function of living cells
• Viral genome contains the blueprint for viral
  replication but is decoded by molecular
  machinery of the cell

3
Key Words

• Pathogenesis effects of virus on host
• Pathogenicity genetic members of a specific
  virus to cause disease and spread thru the
  population
• Genotype genetic make up of virus
• Virulence severity of the viral disease

4
Virulence

• Dependent on
• expression of viral genes
• physiological response of infected individual to
  pathogenic determinants
• response of population to presence of virus
• also affected by general health and genetic
  makeup of infected population, societal and
  economic factors

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More Key Words

• Avirulent or apathogenic strains viruses that
  have mutations that make genes not able to cause
  symptoms wont cause disease but will lead to
  immunity against virulent strains
• people and animals may respond differently to the
  same virus dependent on dose, route of
  infection, immune system, etc
• Epidemiology study of infectious agents in
  humans
• Epizoology study of infectious agents in animals

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Interaction between Virus and Host

• Viruses respond to the environment to become
  wholly or partially resistant
• Viruses in the host can be influenced by
  location, social and economic factors,
  availability of medical attention and
  technology.
• HIV if it was 100 years ago, before traveling
  and large mobilization of people, the virus may
  have stayed isolated and been eradicated by
  depleting the population affected by it

7
History of Virology

• Study of virus is important for more than to
  counteract the discomfort caused by the virus
• Can be used to understand basic info about
  cellular processes since viruses take advantage
  of these processes
• Started with the study of bacteriophage (virus of
  bacteria) that killed E.coli
• Also helpful in determining the immune response
  and immunity to viruses cell-cell signaling
• Help to develop palliative treatments of
  metabolic and genetic diseases humans, plants
  and animals

8
Impact of Viral Disease on Human History

• Egyptian mummies have evidence of genital
  papillomas and polio
• Smallpox now eradicated used to wreak havoc
  in humans, now considered a threat again as an
  agent of bioterrorism
• Europeans are responsible for spreading measles
  into areas that they conquered
• Spanish influenza in 1918-1919 killed millions
  world-wide, not sure yet why this virus was so
  virulent

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Impact of Viruses on the Future

• A virus may evolve so that it becomes more
  devestating
• sequela long term effect of the infectious
  disease
• Viruses that could become more lethal or cause
  more problems are yellow fever, equine
  encephalitis, dengue fever, Ebola, Rift Valley
  fever, hantavirus and the new coronavirus that
  was the causative agent of SARS (severe acquired
  respiratory syndrome)
• Viruses can also cause decimation in animals and
  plants that lead to severe financial losses

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Impact of Evolution Impact of the Virus-Host
Interaction

• Viruses have had an impact on the host evolution
• Viruses can move genes from 1 host to another
• Interferon is a cell-based antiviral made in the
  response to the virus, not found in cells that
  are not infected

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Two Major Examples

• Parasitic wasps lay eggs in caterpillars and the
  caterpillar kills the wasp egg wasp that is
  infected with a polydnavirus can prevent the
  caterpillar from killing the eggs wasp keeps the
  virus in the ovaries and then pass on with the
  new egg
• Chlorella virus is found in fresh water and can
  only infect free algae, cant do this because the
  algae is semisymbiotic with a specific
  paramecium, the selective pressure of the virus
  has forced a symbiotic relationship otherwise the
  algae will be killed by virus

12
Origin of Viruses?

• Cant rely of fossils but some genetic sequences
  are similar to virus, also retroviruses integrate
  into chromosomes and may be part of organism
  forever
• Can find ancient relics of retroviruses in some
  hosts
• Viruses have no ribosomal-like genes, no enzymes
  for energy metabolism viruses didnt evolve
  from free living cells no evidence as there is
  for mitochondria and chloroplasts
• DNA polymerases in viruses are similar to other
  polymerases in gene sequences and function

13
Viruses as Constructive Forces

• Viruses have driven the understanding of
  biological mechanism
• Can use viruses of animal and plant pest to
  control them such as myxoma virus used to
  decrease the European rabbit predation in
  Australia
• Used as therapeutic against against some disease
  and may eventually have a role as prophylactic
  vaccines

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Smaller than Viruses?

• Prions are smaller, composed only of protein that
  has an unusual folding pattern
• Defective viral particles have lost a coat
  protein or replication gene from the genome
• requires a helper virus to become infective by
  replacing the missing piece
• Hepatitis D is an example, cant replicate or
  complicate hepatitis without Hepatitis B being
  present
• RNA pathogen or viriod infects plants but
  doesnt make a protein
• These may mean that viruses came from cells and
  that viruses had multiple origins