Viral genomes can evolve very rapidly for a number of different reasons:

• High rates of mutation – RNA and ss-DNA are less stable and viruses lack proofreading enzymes
  

• Short generation times – Evolution occurs across successive generations and viruses have a typical infectious cycle of ~8–72 hours
  

• Large population sizes – Each infectious cycle can produce large quantities of new virions (>100)
  

Examples of viruses that undergo rapid evolution are influenza viruses and human immunodeficiency virus (HIV)

• Influenza viruses have an infectious cycle of 6 – 12 hours and may produce populations of up to 10,000 virions
  

• HIV has one of the highest mutation rates of any virus (~0.34 mutations per replication cycle)
  

Mechanisms of Viral Evolution

There are two different mechanisms by which viruses may evolve to avoid immune detection and destruction

• Viral evolution can occur progressively via antigenic drift or spontaneously via antigenic shift
  

Antigenic Drift:

• Antigenic drift occurs when the accumulation of point mutations lead to a gradual change in the surface proteins of the virus
  

• Consequently, antibodies produced against the original virus may no longer be able to recognise the new viral strain
  

• Antigenic drift can lead to seasonal reoccurrences of a virus and require new vaccinations (booster shots) to confer protection
  

Antigenic Shift:

• Antigenic shift occurs when cells get infected with multiple viral strains and the gene segments from the two strains get reassorted
  

• This results in a sudden dramatic change in antigenicity and a new viral sub-type is formed to which an entire population is susceptible
  

• Antigenic shift can result in new viral sub-types that may cross species barriers and lead to massive outbreaks (i.e. pandemics)
  

Treatment of Viral Diseases

Rapidly evolving viruses are able to evade detection by an organism’s immune system and consequently cause disease

• Vaccines need to be constantly changed and updated to remain effective against different viral strains
  

• Infectious individuals may need to be isolated (quarantined) to limit the spread of viral infection
  

• Contact tracing and health related databases may be used for monitoring and predicting disease spread
  

• Viral propagation may be reduced by targeting vectors of viral delivery (e.g. via water chlorination, fumigation)
  

• Disease-resistant vectors may be introduced via genetic engineering (e.g. sterile mosquitoes)
  

• Social strategies may commonly involve government legislations designed to support scientific interventions
  

  • Education programmes may be used to inform the public of associated risks (to help minimise exposure)

  • Laws may be passed to ensure public compliance (e.g. customs protocols or vaccination mandates)