This master infector gets infected, in turn, by WO. WO is a bacteriophage, or phage for short—a virus that specializes in infecting bacteria. It can actively make many copies of itself within Wolbachia, eventually bursting out with fatal results. Alternatively, it can insinuate its DNA into Wolbachia’s genome, literally becoming part of its host. As the bacterium reproduces, it copies its genome and so copies WO.

WO can also switch between these two strategies. If two strains of Wolbachia infect the same insect, WO can awaken and violently burst out of one strain, only to infiltrate the other and gently conceal itself. No surprise then that it is present in the vast majority of Wolbachia strains—a nigh-omnipresent virus lurking in the genome of one of the planet’s most widespread microbes.

Sure, WO is cool, but the Bordensteins—a husband-and-wife team—had been studying it for around 15 years, and frankly, they were getting bored. “We were losing interest because we had answered most of the big questions about it,” says Seth. But one vexing mystery remained.

Wolbachia lives inside the cells of its animal hosts. And WO can burst through not just Wolbachia but the surrounding animal cell, too. Somehow it can punch its way through two sets of barriers—one bacterial, and one animal. Once it’s out, to find a new host, it must punch its way back into another insect cell and another Wolbachia. All viruses are masters of escape and infiltration, but WO must be doubly so. How does it manage?

To find out, the Bordensteins sequenced the phage’s genome. It was largely unsurprising, containing all the expected genes for infecting bacterial hosts and building new viruses. But amid these usual suspects, the Bordensteins noticed a weird and previously unnoticed cluster of genes, taking up a full third of the WO genome.

“These genes are all very strange,” says Sarah. They’re definitely part of the phage, because they aren’t found in Wolbachia strains that don’t contain WO. But they don’t look like phage genes at all—or even bacterial ones. Instead, they had several hallmarks of animal genes, and specifically those of the spiders, insects, and other invertebrates that Wolbachia infects.

For example, the virus contained part of the gene for latrotoxin—the chemical in black widow spider venom. The toxin punches holes through the cells of victims, causing their innards to leak fatally outwards. “There hasn’t been another case of a latrotoxin being found outside of spiders,” says Seth.

It’s possible that the spiders got the latrotoxin gene from the virus, or that the two evolved their copies independently. But by comparing the various versions of latrotoxin, the Bordensteins think that it’s most likely that the virus got the gene from spiders. It certainly had the right opportunity, since Wolbachia, its host microbe, does indeed infect black widows. The phage could have picked up spider DNA directly from the creature’s own cells. Or Wolbachia could have picked up spider DNA and then transferred it to the phage. Or other as-yet-unidentified viruses and bacteria could have acted as intermediaries.