UBIQUITOUS INFECTIONS. “A rough calculation reveals that there are many billions of chronic virus infections in humans, with each of the ∼6.75 billion of us harboring ∼8–12 chronic infections. Some of these viruses are so common that one would be hard put to find uninfected control patients for studies to identify viral associations with disease or effects on the “normal” immune system.”
http://www.sciencedirect.com/science/article/pii/S0092867409007831