But the discovery of giant, ancient viruses suggests that today's contagions might actually have evolved from whole cells that shrank down, shedding the bits and bobs they didn't need while surviving and propagating within infected hosts.
Whatever their true origin is, it's not surprising to find evidence of gene swapping in a virus's DNA - that's rather the point of a virus.
But the new study represents the first time that a virus appears to have borrowed DNA from a kingdom other than the one it infects.
WO infects a bacteria called Wolbachia that in turn infects arthropods - it doesn't infect the arthropod bugs themselves, but the bacterial bugs inside them. No virus had ever been found to cross kingdoms in the creatures it infects.
Instead of simply showing a history of interaction with Wolbachia, one-third of WO's genome seems to show some intermingling with the animal kingdom.
The virus even contains part of the gene for latrotoxin, the chemical that gives the black widow spider venom its punch.
"There hasn't been another case of a latrotoxin being found outside of spiders," Vanderbilt University's Seth Bordenstein, who co-authored the paper along with his spouse and lab partner, Sarah Bordenstein, told the Atlantic.
They found other bits of genes associated with animal cells, too: ones related to the sensing of pathogens (that then help lead to cell death to kill off the invader) and ones used to evade immune system detection.
If the Bordensteins are correct in their findings - other researchers will have to repeat the analysis to ensure that no contamination has led them astray - the next big question is why WO is unique in the viral world.
It could be because of Wolbachia's way of merging with its animal hosts. The bacteria wraps itself up in the cellular membranes of the animal cells it infects. To get out of one Wolbachia cell and into another, WO actually has to burst through both a bacterial membrane and an animal one.
"We suspect it makes pores in the membranes of the arthropod cells that surround Wolbachia, thereby allowing the phage to overcome both the bacterial and arthropod membranes that surround it. That may be how it uses some of these proteins," Seth Bordenstein said.
So while it doesn't make animals sick, WO does have to travel through animal cells without being destroyed. That could explain how it's been able to pick up arachnid and insect DNA, and why having genes related to this other kingdom of creatures could help it slip through incognito, just as an animal-infecting virus might.
As if this Inception-like DNA swapping wasn't mind-blowing enough, the Bordensteins say there's a chance - albeit a small one - that the transfer went the other way around. It's possible that spiders and other bugs first got these genes from viruses.
