The myxoma virus quickly evolved. The strain that had initially been used was almost inescapably lethal, killing virtually every rabbit it infected. But virologist Frank Fenner discovered that, within a few years, this strain had been replaced with milder ones, which killed less rapidly and frequently.

These events provided an unprecedented view of how viruses evolve in the wild. They’ve also permeated into the popular consciousness, creating an intuitive sense that lethal viruses eventually evolve into milder forms, which are less likely to completely wipe out their hosts. But “the notion that everything’s heading toward a state of long-term co-existence and happiness is not always the case,” says Andrew Read, an evolutionary biologist based at Pennsylvania State University. “There are plenty of examples where the virus has got nastier over time.”

And as it happens, myxoma is one such example. “It went from exceptionally nasty to just nasty, and now has turned round and cranked up the nastiness again,” Read says.

The virus was never entirely defanged. After its release in 1950, it went from killing more than 99 percent of rabbits to killing around 75 percent of them, or under 50 percent in some cases. In response, the rabbits evolved resistance, shrugging off strains that would once have finished them off. And that relaunched the arms race between myxoma and rabbits, prompting the virus to evolve its own countermeasures, which it still deploys today.

Read worked out how it responded by teaming up with Peter Kerr, who had collected and stored myxoma samples from the last several decades. By exposing lab rabbits to these archived strains, the team showed that by the 1990s, the virus had gained a new ability: It could completely shut down a rabbit’s immune system. This stops the animals from effectively clearing the virus. Inadvertently, it also means the bacteria that normally live peacefully in the rabbits’ bodies run amok, spreading through their internal organs and causing septic shock. These rabbits never develop the skin tumors or any of the classic symptoms of myxomatosis. Instead, they die from massive and sudden infections. Their lungs fill with fluid and they start bleeding uncontrollably.

These immune-suppressing strains might have emerged as early as the 1970s, and they’re circulating broadly now. Still, their effects are hard to spot in wild rabbits, which still die from the same kinds of symptoms as they used to. That’s because their genetic resistance partly counteracts the virus’s new ability, which only becomes clear when it infects lab animals that have no history of coevolving with this virus. The wild rabbits started to resist the virus, the virus started to kill them in a new way, and neither side gained any ground. “It’s like a duck in a stream, paddling like crazy under the water and not getting anywhere,” says Read.