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Viruses can evolve to be more deadly

July 1, 2021 GMT

CLAIM: No virus has ever mutated to become more lethal. As viruses mutate, they become less lethal.

AP ASSESSMENT: False. There are documented cases of viruses becoming more deadly.

THE FACTS: As the spread of coronavirus variants raise new public health questions, social media users are sharing misinformation about how viruses mutate.

A post on Facebook reads, “In the history of virology, there has never, EVER, been a viral mutation that resulted in a virus that was MORE lethal. As viruses mutate, they become more contagious/transmissible and LESS lethal.”

But in fact, there have been cases of viruses that mutated to become more deadly.

“That claim as a whole is just nonsense,” said Troy Day, a professor of mathematics and biology at Queen’s University in Canada, who has studied the ways infectious diseases, including coronavirus, can evolve.

Some examples of viruses that became more deadly over time include those that developed drug resistant variants, and animal viruses such as bird flu, which were harmless to humans initially but then mutated to become capable of killing people, according to Dr. Amesh Adalja, a senior scholar at Johns Hopkins University’s Center for Health Security.

“Flu viruses have developed resistance to certain antivirals that make them more difficult to treat, and therefore make them more deadly,” Adalja said, also noting the same has happened with HIV and certain Hepatitis C strains.

Viruses constantly mutate as they copy themselves. Some mutations may not make meaningful changes, while others can give the virus new characteristics. While early scientific theories suggested that as viruses evolved, they would become more contagious and less lethal to keep spreading, over time the scientific community has acknowledged that’s not always the case.

“Becoming more transmissible and less lethal are absolutely what’s best for the pathogen,” said Day. “But the problem is that it’s not always possible, and in many instances is never possible, to be more transmissible and also less lethal.”

Day said there are documented cases of animal viruses that evolved over time to become more lethal, including myxoma virus in rabbits and Marek’s disease in chicken.

Some viruses provoke severe symptoms in their hosts that make it easier to transmit the virus to others. But those same symptoms can wind up killing the hosts.

Adalja said one example is Ebola, a deadly virus that spreads through the blood and body fluids of infected people. Another example is norovirus, which causes diarrhea and vomiting, and leads to hundreds of deaths each year in the U.S.

“The virus, speaking anthropomorphically, just wants to spread and have its genes replicated,” said Adalja. “If the best way for it is to spread by causing severe symptoms it will continue to do that.”


This is part of The Associated Press’ ongoing effort to fact-check misinformation that is shared widely online, including work with Facebook to identify and reduce the circulation of false stories on the platform.

Here’s more information on Facebook’s fact-checking program: