A virus is a box of chemicals, without the structures of a basic cell. It cannot metabolize or replicate on its own. In order to reproduce, it must invade living cells. Viruses infect bacteria and plants, reptiles, fish, birds, and mammals. They have been so intertwined with our own evolution that over millennia some became incorporated into our own genetic code. Nestled into the long strands of our DNA are sequences that originated from ancient viruses. Their genetic code was so bound with ours that the virus became a harmless part of us, freeloading off human cells as they reproduced.
