It's all a part of....

"Some viruses, like Ebola or the new avian influenza, are basically runaway replicators, effectively burning their own life bridges in the process. But the majority, as Villarreal puts it, strive 'to persist, not make a lot.' Those that do persist eventually become both stable within, and staples of, evolution. The overwhelming majority of viruses are not harmful to their hosts. Each of us is infected with a huge array of viruses. The human genome, considered as a mass, contains more retrovirus sequences than actual genes.

"'They're not doing anything,' says Villarreal. 'They're just persisting. And they were around long before humans evolved. The better part of the human genome is composed of viral DNA. That's true of nearly all eukaryotes, and the more complicated the organisms, the more of those sequences you have. We aren't sure exactly what they all do, but they are part of our genetic identity, this stuff we dismiss as junk. "Junk" and "parasite" are both words that will get you into a fight if you use them improperly. And yet they are where all life's creativity lies—its very origins.'"

-From Unintelligent Design