A rather surprising amount of the human genome is made up of debris from ancient viral infections. Some of that genetic material does nothing, so far as we know, some can trigger cancer under certain conditions, and some has become essential to our survival. The cells that form the outer layer of the placenta for a human fetus bind to each other using a gene that originated, long ago, from a virus. Though many viruses cannot reproduce without us, we ourselves could not reproduce without what we have taken from them.
