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Vivian Lam
By Vivian Lam December 8, 2021

A claim that viruses and other pathogens always evolve to become less lethal is false

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  • A number of factors influence the way the virulence of a virus — that is, how harmful it is to its host — affects its ability to survive and spread; in some cases, higher virulence may help a virus survive and spread.

  • Many viruses have evolved to become more lethal over time.

As scientists work to uncover the characteristics of recently discovered COVID-19 variant omicron, social media posts are claiming that it represents a diminished risk to the public.

"Pathogens evolve to become less, not more, virulent over time," says one Facebook post. "If pathogens evolved to become more virulent with time they’d destroy their hosts which they depend on to live and you wouldn’t be here to read this." The post cites HIV in Uganda and Ebola as examples of viruses that have become less virulent with time.

This post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

The claim makes a broad generalization about pathogens that’s not supported by science. It has been well-documented that pathogens can evolve to be more virulent. And many viruses, including HIV and Ebola, have in fact become more lethal over time.

"You can’t just say it’s going to become nicer — that somehow a well-adapted pathogen doesn’t harm its host," Andrew Read, an evolutionary microbiologist at Penn State University, said in an interview. "Modern evolutionary biology, and a lot of data, shows that doesn’t have to be true. It can get nicer, and it can get nastier."

How viral mutation works

Viruses, which are composed primarily of genetic material encased in either a protein or lipid shell, can’t replicate on their own. They rely on their hosts to provide the machinery to copy their genetic material and produce the proteins they need to propagate. During that process, mutations may be introduced into their genetic material that can make them more or less effective at surviving and spreading. These mutations also affect the virus' virulence, or its capacity to harm its host.

Viruses that are given enough time to replicate and spread are more likely to develop a mutation that aids their survival. Which types of mutations are helpful to a virus, however, can be difficult to predict.

What scientists want to understand about any given pathogen are the selective pressures, or external factors that affect their ability to survive, that push them to become more or less infectious and harmful over time.

The claim on Facebook aligns with a theory attributed to the late-19th-century pathologist Theobald Smith, who observed that cattle repeatedly exposed to a tick-borne pathogen developed more moderate infections compared to first encounters.

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A subsequent theory, the trade-off model, posits that pathogens evolve by balancing their need to propagate using host resources against the risk of doing too much harm to their hosts.

These models, however, are subject to debate among evolutionary biologists, and available studies provide only partial support for them. Other factors, including vaccines and how many hosts are available to be infected, further complicate the external pressures that determine how a virus might evolve. HIV, for example, adapts very quickly to local conditions — while it may be weakening in one area of the world, it may become more lethal in another.

Many viruses have gotten deadlier over time. Some, including influenza and HIV, have developed drug-resistant variants. Others, including Ebola and Zika, have adapted to human hosts by evolving in ways that maximize their ability to infect people. Even the myxoma virus, a rabbit pathogen often cited as an example of declining virulence, has been shown to become more lethal over time.

A virus doesn’t necessarily need to keep its host alive in order to continue spreading and replicating its genetic material. And notably, the virus that causes COVID-19 is most transmissible before symptoms set in, so its harmfulness to its host may not be as strong an influence on how it evolves as the trade-off model suggests.

"The virus, speaking anthropomorphically, just wants to spread and have its genes replicated," said Dr. Amesh Adalja, an infectious disease specialist at Johns Hopkins University, in an interview. "If the best way for it is to spread by causing severe symptoms, it will continue to do that."

Our ruling

A Facebook post claims that pathogens evolve to become less virulent over time, because otherwise, "they’d destroy their hosts which they depend on to live."

Viruses can evolve to become both more or less virulent over time, and do not necessarily require the survival of their host to maximize their transmission. This post cherry-picks examples to make a generalization that ignores the wide swath of epidemiological and laboratory evidence showing that pathogens can evolve to become more lethal.

We rate this statement False.

CORRECTION: This story was updated Jan. 11, 2022, to remove two sentences about malaria. It is a disease caused by a parasite, not a virus.

Our Sources

The New York Times, Omicron: What We Know About the New Coronavirus Variant, Dec. 4, 2021

Facebook post, Nov. 28, 2021

Nature Reviews Genetics, The phylogenomics of evolving virus virulence, Oct. 10, 2018

Smithsonian Magazine, How Viruses Evolve, Jul. 17, 2020

Caltech Science Exchange, What is a virus?, accessed Dec. 5, 2021

North Carolina State University, A Primer on Coronavirus, Variants, Mutation and Evolution, Mar. 4, 2021

Infection and Immunity, Host-Pathogen Interactions: Redefining the Basic Concepts of Virulence and Pathogenicity, Dec. 18, 2020

Viral Pathogenesis, Viral Evolution: It Is All About Mutations, Feb. 12, 2016

History and Philosophy of the Life Sciences, Why do parasites harm their host? On the origin and legacy of Theobald Smith's "law of declining virulence"--1900-1980, 2012

Journal of Evolutionary Biology, Virulence evolution and the trade-off hypothesis: history, current state of affairs and the future, Jan. 19, 2009

Evolution, Virulence-driven trade-offs in disease transmission: A meta-analysis, Feb. 8, 2019.

Parasitology, The adaptive evolution of virulence: a review of theoretical predictions and empirical tests, Aug. 25, 2015

NAM Aidsmap, HIV has become more virulent over time, not less, European study finds, Dec. 10, 2014

Nature Education, Genetics of the Influenza Virus, 2008

Nature Communications, Natural selection favoring more transmissible HIV detected in United States molecular transmission network, Dec. 19, 2019

Cell, Human Adaptation of Ebola Virus during the West African Outbreak, Nov. 3, 2016

Cell, Ebola Virus Glycoprotein with Increased Infectivity Dominated the 2013–2016 Epidemic, Nov. 3, 2016

Frontiers in Microbiology, Contributions of Genetic Evolution to Zika Virus Emergence, May 6, 2021

Proceedings of the National Academy of Sciences, Next step in the ongoing arms race between myxoma virus and wild rabbits in Australia is a novel disease phenotype, Aug. 29, 2017

Proceedings of the National Academy of Sciences, Virulence and competitive ability in genetically diverse malaria infections, May 24, 2005

JAMA Internal Medicine, COVID-19 Transmission Dynamics Among Close Contacts of Index Patients With COVID-19: A Population-Based Cohort Study in Zhejiang Province, China, Aug. 23, 2021

The Conversation, How the coronavirus escapes an evolutionary trade-off that helps keep other pathogens in check, Jun. 17, 2020

AP, Viruses can evolve to be more deadly, Jul. 1, 2021

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