Ancient retroviruses inserted their genetic material into our ancestors' germline cells, and that DNA was copied into every descendant, so roughly eight percent of the human genome is viral in origin. Some of this inherited viral code has been repurposed by the body, for example to build the placenta.
Instead of viewing viruses only as agents of disease, they can be seen as mobile carriers of genes that move DNA between hosts and species. Through this horizontal transfer they have repeatedly introduced new functions into other organisms, acting as a creative force in evolution rather than merely a destructive one.
Many dangerous human viruses, such as HIV and SARS, originated in animals and jumped to people when ecological contact increased, for instance through hunting or wildlife trade. Because mutation lets a virus adapt to a new host, growing human-animal contact makes the emergence of previously unknown epidemics an ongoing risk.