To stay healthy, your cells must continuously perform a wide variety of chemical tasks. Your DNA genes, in the nucleus of every cell, help orchestrate this activity. A cell uses its genes as a kind of biochemical cookbook: whenever it has to do anything, a cell looks up the recipe in its DNA. It then uses this recipe to cook up whatever proteins it needs to get the job done. Your cells are hard at work right now, creating thousands of tailor made proteins for thousands of different biochemical jobs. Sometimes, however, things can go wrong. When a virus attacks, it injects its own DNA into one of your cells. Like your own genes, the viral DNA contains recipes. Unfortunately, these recipes are only good for making new viruses.