The Evolving Nature of Disease

Spring Biology

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.

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.

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.

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.

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.