In the invisible, parallel world of Earth's they kill half the bacteria in the ocean every day, and invade a microbe host 10 trillion times a second around the world. There are 10 billion trillion, trillion viruses inhabiting Planet Earth, which is more stars than are in the Universe -- stacked end to end, they would reach out 100 million light years.
Over tens, hundreds and millions years, our ancestors have been picking up retroviruses (HIV is a retrovirus) that reproduce by taking their genetic material and inserting it into our own chromosomes. There are probably about 100,000 elements in the human genome that you can trace to a virus ancestor. They make up about 8 percent of our genome, and genes that encode proteins only make up 1.2 percent of our genome making us more virus than human.
Occasionally, a retrovirus will end up in a sperm cell or an egg and insert its genes there, which then may give rise to a new organism, a new animal, a new person where every cell in that body has got that virus.
Via Dr. Stefan Gruenwald