�������t¶��

Scientists killed hundreds of millions of them by introducing the myxoma virus — the deadliest vertebrate virus — but as Carl Zimmer wrote in June, the rabbits adapted and kicked off an evolutionary arms race.

It looked like a benign tumor, called a myxoma, Minella said.

But the myxoma virus — fatal to millions of Australian rabbits — is a textbook example of a virus that did not get milder over time.

For many virologists, the best example of this unpredictability is a pathogen that has been ravaging rabbits in Australia for the past 72 years: the myxoma virus.

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.