Scientists' research has revealed that viruses are by far the most abundant life forms on Earth. There are a million times more viruses on the planet than stars in the universe. Viruses also harbor the majority of genetic diversity on Earth. Scientists are finding evidence of viruses as a planetary force, influencing the global climate and geochemical cycles. They have also profoundly shaped the evolution of their hosts. The human genome, for example, contains 100,000 segments of virus DNA.