They are found anywhere that life is present and far outnumber all forms of life on earth—even bacteria. A liter of seawater contains over 100 billion virus particles, and one kilogram of dried soil somewhere in the region of a trillion.[18] The total number on the planet is estimated at about 1031—that is, one followed by thirty-one zeros.[19] But only about 220 types of virus are known to be capable of infecting humans.[20]
