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An Introduction to Why We Get Sick



A primary focus of evolutionary medicine is to understand why we get sick. In some cases, this may be a question of frequency - why hasn't natural selection eliminated our risk for this disease? In other cases, this may be a question of recent changes to disease risk - how can evolution help us understand why certain disease states are becoming more common? Sometimes this question simply pertains to overall health - why aren't we healthier, or why don't we live longer?

Evolutionary answers to these questions typically require understanding six principles.

  1. Evolutionary Mismatch
    • Traits that evolved in a previous environment may lead to dysfunction after a rapid environmental change.
  2. Coevolutionary arms-races
    • Pathogens continually evolve and adapt alongside the evolution of host defenses or antimicrobial substances.
  3. Symptoms caused by our bodies own evolved defenses
    • Sometimes, what we think of as being sick is actually our bodies own defense system fighting off a dangerous infectious agent.
  4. Evolutionary constraints
    • Certain traits may seem beneficial, but this does necessarily mean evolutionary processes will favor those traits. Constraints often prevent the evolution of further adaptations.
  5. Trade-offs
    • Traits are interrelated, so sometimes an improvement to one trait requires making another trait worse.
  6. Reproduction at the cost of health
    • Traits that increase the likelihood of disease can be selected for so long as they lead to greater reproductive fitness.

One or more of these concepts typically underlie evolutionary explanations for any given disease vulnerability. For this reason, this list serves as a useful conceptual framework for students in EvMed. As students move from novices in EvMed toward expertise, they should increasingly think about disease in light of these different explanations.

Principles this example illustrates:

Additional resources:




Journal articles:

  1. Nesse, R. M., & Dawkins, R. (2010). Evolution: Medicine’s most basic science. In D. A. Warrell, T. M. Cox, J. D. Firth & E. J. J. Benz (Eds.), Oxford Textbook of Medicine, 5th edition (pp. 12-15). Oxford: Oxford University Press.
  2. Gluckman, P. D., Low, F. M., Buklijas, T., Hanson, M. A., & Beedle, A. S. (2011). How evolutionary principles improve the understanding of human health and disease. Evolutionary applications4(2), 249–263.

Teaching materials:

Why We Get Sick Card Sort Activity:

Activity Materials
Slides (for use or modification)

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