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The Evolving Nature of Disease

Spring Biology

Shining Evolution’s Light of Disease

  • Immunizations are an important tool of modern medicine.
    • Protection is long-lasting for many diseases, or require redosage, like the flu.
  • Repeated immunization to the flu: evolution of the influenza virus.
  • Evolution: change in the heritable characteristics of a population over successive generations.

How Does Influenza Evolve?

  • Evolution cannot occur without a source of genetic variation - mutation.
  • During an influenza infection, the flu reproduces itself; sometimes, an error occurs.
  • Mutations give rise to new traits that can be advantageous.

Spillovers, Reassortment, and Pandemics

  • Human populations are not only plagued by annual outbreaks of seasonal influenza, but also by global influenza pandemics.
  • Viral subtypes are given names based on the cahracteristics of their protein coats (H1N1, etc.)
    • Arises from a spillover from a wild animal population via reassortment.
    • Reassortment - two versions of the influenza virus infect a common host and swap RNA segmetns to produce a novel flu strain.
  • Ebola, COVID-19, HIV/AIDS were all once restricted to animals before they spilled over to infect and spread through human populations.

What Makes a Disease Successful?

  • Many heritable traits that make pathogens successful are analogous to traits that aid non-pathogenic organisms.
  • For the pathogen, advantage or disadvantage of a trait depends on interaction with the host.

Evolution Favors the Optimal Level of Virulence

  • Why do pathogens ever evolve to be harmful to their hosts?
  • Harm caused to hosts by pathogens: virulence.
    • Pathogens do benefit from being virulent, but harm is a byproduct of infection, not a goal.
  • Pathogens consume their host’s resources to reproduce, and the host is inevitably harmed in the process.
  • Virulent diuseases are expected to be more transmissable.
  • Evolution by natural seelction is expected to favor intermediate virulence that maximizes the spread of the disease.