The community of viruses is staggeringly vast. Occupying every conceivable biological niche, from searing undersea vents to frigid tundra, these enigmatic invaders, hovering between inert matter and life, circumnavigate the globe in the hundreds of trillions. They are the most abundant life forms on earth.
Viruses are justly feared as ingenious pathogens, causing diseases in everything they invade, including virtually all bacteria, fungi, plants and animals. Recent advances in the field of virology, however, suggest that viruses play a more significant and complex role than previously appreciated, and may be essential to the functioning of diverse ecosystems.
We now know that humans contain roughly 100,000 pieces of viral DNA elements, which make up around 8 percent of our genome. Speculation on the role of these ancient viral fragments ranges from protection against disease to increasing the risk of cancer or other serious illnesses, though researchers acknowledge they have barely scratched the surface of this enigma.
Read more at Arizona State University