http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1006292 A graph of how many people in the sample each virus is found in, and the estimate of abundance per 100000 human blood cells. Some viruses can be in a latent form, integrated into human DNA, to remain latent for long periods. This shows how many are integrated versus abundance. Caveats: This study was not aimed at finding either rare, or the similar population of RNA viruses that exist. It also shows the problem with contamination, in that most of the viruses found (70) were lab containments, with some from well understood and expected sources used for calibration, but others from a wide variety of lab sources.