An estimated 10 nonillion (10 to the 31st power) individual viruses exist on our planet—enough to assign one to every star in the universe 100 million times over.

Viruses infiltrate every aspect of our natural world, seething in seawater, drifting through the atmosphere, and lurking in miniscule motes of soil. Generally considered non-living entities, these pathogens can only replicate with the help of a host, and they are capable of hijacking organisms from every branch of the tree of life—including a multitude of human cells.

Yet, most of the time, our species manages to live in this virus-filled world relatively free of illness. The reason has less to do with the human body’s resilience to disease than