There are some 320,000 unique mammalian viruses, according to estimates by virologists, and likely exponentially more existing on the planet today. Determining an accurate number would require billions more dollars and a great deal more manpower than is currently given to the study of viruses. Though a handful of viruses live in and on our bodies at all times—known as the virome—not all of them make us ill; just as often, they lie dormant. Many virus functions remain mysterious to scientists, such as how they enter a cell or replicate, though existing test advances, like the VirScan blood test, can tell you any infection you’ve ever had.
