Do viruses always mutate to become less lethal over time?

“That claim as a whole is just nonsense,” said Troy Day, a professor of mathematics and biology at Queen’s University in Canada, who studies how infectious diseases evolve.

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.

Day said there are documented cases of animal viruses in nature becoming more deadly, including myxoma virus in rabbits and Marek’s disease in chickens.

Some viruses mutate to become resistant to drugs, which can make them more lethal, said Dr. Amesh Adalja, a senior scholar at Johns Hopkins University’s Center for Health Security. Adalja said some animal viruses, such as bird flu, start out as harmless to humans but then mutate to become capable of killing people.