“Viruses are not just these threatening or annoying parasitic agents,” he added. “They’re the creative front of biology, where things get figured out, and they always have been.”

Researchers are deeply impressed by the depth and breadth of the viral universe, or virome. Viruses have managed to infiltrate the cells of every life form known to science. They infect animals, plants, bacteria, slime mold, even larger viruses. They replicate in their host cells so prodigiously and stream out into their surroundings so continuously that if you collected all the viral flotsam afloat in the world’s oceans, the combined tonnage would outweigh that of all the blue whales.

Not that viruses want to float freely. As so-called obligate parasites entirely dependent on host cells to replicate their tiny genomes and fabricate their protein packages newborn viruses, or virions, must find their way to fresh hosts or they will quickly fall apart, especially when exposed to sun, air or salt.

“Drying out is a death knell for viral particles,” said Lynn W. Enquist, a virologist at Princeton.

How long shed virions can persist if kept moist and unbuffeted — for example, in soil or in body excretions like blood or vomit — is not always clear but may be up to a week or two. That is why the sheets and clothing of Ebola patients must be treated as hazardous waste and surfaces hosed down with bleach.

Viruses are masters at making their way from host to host and cell to cell, using every possible channel. Whenever biologists discover a new way that body cells communicate with one another, sure enough, there’s a virus already tapping into exactly that circuit in its search for new meat.