Viruses make their living by breaking into cells and using the machinery and energy in the cell to reproduce. Once inside, some viruses immediately hijack the cell and make copies of themselves which burst out into the world to infect new cells. Other viruses take a staid approach, though. Instead of taking over the cell, they quietly slip a copy of their genes into its DNA. When the cell divides, it copies the newly acquired viral genes along with the rest of its genome.