A virus is a parasite, having no cell machinery to reproduce itself or make its own proteins. It has to wait until it invades a living cell where it takes over, forcing the cell to making thousands of copies of it. These burst through the cell membrane and are released, killing the cell in the process. Once released, each copy of the virus has to seek the opportunity to invade another cell. The virus's DNA can also insert itself into the host cell's DNA, becoming a permanent resident.
Viruses infect everything that lives, including all animals, plants and even bacteria.
A clever virus, such as the retrovirus HIV, kills the host cell very slowly so that it can survive as long as possible. But sometimes accidents happen. No fossils of viruses have ever been found. Nevertheless, they are thought to be very ancient organisms, possibly primeval. It mutated and infected humans when men hunting monkeys for research purposes came into contact with monkey blood or bloodied tissue.