Paul W. Ewald

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Paul W. Ewald (born c.1953) is an evolutionary biologist, specializing in the evolutionary ecology of parasitism, evolutionary medicine, agonistic behavior, and pollination biology. He is the author of Evolution of Infectious Disease (1994) and Plague Time: The New Germ Theory of Disease (2002), and is currently director of the program in Evolutionary Medicine at the Biology Department of the University of Louisville.

Ewald is known for his "theory of virulence”, suggesting that "the deadlier the germ, the less likely it is to spread",[1] and his theory that many common diseases of unknown origin are likely the result of chronic low-level infections from viruses, bacteria or protozoa.

Education and career

Ewald received his BSc in 1975 from the University of California, Irvine, in biological sciences and his PhD in 1980 from the University of Washington, in zoology, with specialization in ecology and evolution.[2] He was formerly a professor of biology at Amherst College, and is currently director of the program in evolutionary medicine in the Biology Department of the University of Louisville.[3]


Ewald asserts, along with a growing body of studies, that many common diseases of unknown origin are likely the result of chronic low-level infections from viruses, bacteria or protozoa.[4] For example, cervical cancer can be caused by the human papilloma virus,[5] some cases of liver cancer are caused by hepatitis C or B[6] and the bacteria Helicobacter pylori has been proven to cause stomach ulcers.[7] Ewald argues that many common diseases of currently unknown etiology, such as cancers, heart attacks, stroke and Alzheimer's, may likewise be also caused by chronic low-level microbial infection.[8]

Ewald disagrees with the popular theory that genes alone dictate chronic disease susceptibility. Ewald, whose background is in evolutionary biology, points out that any disease causing gene that reduces survival and reproduction would normally eliminate itself over a number of generations. Ewald says that "chronic diseases, if they are common and damaging, must be powerful eliminators of any genetic instruction that may cause them."[9] One example of this is schizophrenia; patients with this mental illness rarely reproduce. Ewald argues that, just by evolutionary pressures, schizophrenia would have already been eliminated if its causes were strictly genetic; he suggests that in the future, an infectious cause of schizophrenia will be discovered.[10]

Ewald explains that purely genetic causes of chronic disease will persist only if a genetic instruction provides a compensating benefit (for example, the disease sickle cell anemia is caused by a genetic mutation that, in heterozygotes, protects against malaria, which kills millions worldwide each year).[9]

Further evidence for a non-genetic etiology of diseases like schizophrenia, Ewald also points out, comes from concordance studies on identical twins, which measure the percentage of identical twins who both develop a disease. A concordance of 100% indicates a primarily genetic disease, which is not really influenced by environmental factors like infection, nutrition, or toxins. Huntington's disease, for example, has a concordance rate of 100%, indicating a predominately genetic etiology. However, when the concordance rate is lower, this indicates environmental factors like infectious microbes or toxin exposure are playing a causal role. Schizophrenia's concordance is approximately 35-60%, suggesting, says Ewald, that microbes are etiologically involved.[2] Another example is breast cancer: Ewald notes that in the case of identical twins, when one twin develops breast cancer, the other twin has only a 10% to 20% chance of developing the disease, and this concordance rate of just 20% again indicates that environmental factors like infectious microbes or toxins are likely playing large causal roles in breast cancer.

Ewald's curiosity regarding the evolutionary process of infections was sparked by a bad case of diarrhea he had in 1977.[11] His first thought during this bout was that his body was using diarrhea to expel the pathogen and he should avoid anti-diarrheal medication. Looking at the problem from the standpoint of the organism, expulsion was not an evolutionary benefit. The only benefit to the pathogen causing the sickness would be the potential transmission to other hosts; much like the particulate expelled during coughing, diarrhea can be a means of distribution. Another major influence on Ewald's thinking in evolutionary biology terms was the HIV virus, which once caught, initially remains inactive for years thus allowing it to spread, before the chronic disease of AIDS finally manifests, incapacitates, and eventually kills the host.[12]


In 2010, Utne Reader magazine named Ewald as one of the "25 Visionaries Who Are Changing Your World" for his research on the link between infections and cancers.[13]


  • "Like many great ideas in biology, the idea implicating infectious causation in chronic diseases, though simple, has far-reaching implications. It is so simple and so significant, that one would think it would have been recognized by many and would be the starting point for any discussion on the causes of disease. Not yet." — Paul W. Ewald.[9]
  • "If I were going to put my money on it, I would bet that by 2050—hopefully earlier—we’ll have found that more than 80 percent of all human cancer is caused by infection." — Paul W. Ewald.[14]
  • Of Ewald's theory: "It opens our eyes to many quite weird possibilities about disease that most medical scientists, tending to be unaware of current evolutionary thought, don't think of." — evolutionary biologist William D. Hamilton.[citation needed]




See also


  1. ^ Orent, Wendy (16 November 2020). "Will the Coronavirus Evolve to Be Less Deadly? - History and science suggest many possible pathways for pandemics, but questions remain about how this one will end". Smithsonian Magazine. Retrieved 29 November 2020.
  2. ^ a b Proal, Amy (Feb 2008). "Interview With Evolutionary Biologist Paul Ewald". Discover Magazine.
  3. ^ Biology Department Faculty University of Louisville
  4. ^ Plague Time, p.3
  5. ^ Bosch, FX; Lorincz, A; Muñoz, N; Meijer, CJ; Shah, KV (2002). "The causal relation between human papillomavirus and cervical cancer". Journal of Clinical Pathology. 55 (4): 244–65. doi:10.1136/jcp.55.4.244. PMC 1769629. PMID 11919208.
  6. ^ Michielsen, Peter P; Francque, Sven M; Van Dongen, Jurgen L (2005). "Viral hepatitis and hepatocellular carcinoma". World Journal of Surgical Oncology. 3: 27. doi:10.1186/1477-7819-3-27. PMC 1166580. PMID 15907199.
  7. ^ Hadley C (2006). "The infection connection. Helicobacter pylori is more than just the cause of gastric ulcers--it offers an unprecedented opportunity to study changes in human microecology and the nature of chronic disease". EMBO Rep. 7 (5): 470–3. doi:10.1038/sj.embor.7400699. PMC 1479565. PMID 16670677.
  8. ^ Plague Time, p.6
  9. ^ a b c Plague Time, p.56
  10. ^ Plague Time, p.156
  11. ^ Hooper, Judith (Feb 1999). "A New Germ Theory: Part 1". The Atlantic Monthly.
  12. ^ Hooper, Judith (Feb 1999). "A New Germ Theory: Part 2: Antibiotics Against Heart Disease?". The Atlantic Monthly.
  13. ^ "Paul Ewald: Virally Minded". Retrieved 19 October 2010.
  14. ^ Grant, Andrew (Sep 2009). "The Big Idea That Might Beat Cancer and Cut Health-Care Costs by 80 Percent". Discover Magazine.
  15. ^ Ewald., Paul W. (1994). Evolution of Infectious Disease. Oxford: Oxford University Press. ISBN 978-1423734697. OCLC 228117631.
  16. ^ Plague Time
  17. ^ Ewald, P. W. (1991). "Transmission modes and the evolution of virulence : With special reference to cholera, influenza, and AIDS". Human Nature (Hawthorne, N.Y.). 2 (1): 1–30. doi:10.1007/BF02692179. ISSN 1045-6767. PMID 24222188. S2CID 11765810.
  18. ^ Ewald, Paul W. (1995). "The Evolution of Virulence: A Unifying Link between Parasitology and Ecology". The Journal of Parasitology. 81 (5): 659–669. doi:10.2307/3283951. JSTOR 3283951. PMID 7472852.
  19. ^ Ewald, Paul (1996). "Guarding Against the Most Dangerous Emerging Pathogens: Insights from Evolutionary Biology". Emerging Infectious Diseases. 2 (4): 245–257. doi:10.3201/eid0204.960401. ISSN 1080-6040. PMC 2639916. PMID 8969242.
  20. ^ Kaufmann, Stefan H. E (2012-05-24). "Vaccines as evolutionary tools: The virulence-antigen strategy". Concepts in vaccine development. Kaufmann, S. H. E. (Stefan H. E.). Berlin. ISBN 9783110906660. OCLC 859154818.
  21. ^ Ewald, Paul W.; Leo, Giulio De (2002), "Alternative Transmission Modes and the Evolution of Virulence", Adaptive Dynamics of Infectious Diseases, Cambridge University Press, pp. 10–25, doi:10.1017/cbo9780511525728.004, ISBN 9780511525728
  22. ^ Ewald, Paul W. (2002), "Virulence Management in Humans", Adaptive Dynamics of Infectious Diseases, Cambridge University Press, pp. 399–412, doi:10.1017/cbo9780511525728.036, ISBN 9780511525728, retrieved 2018-09-08

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