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But the process of copying genes is imperfect, and it always introduces a few random mistakes, especially in viruses like COVID, influenza, and Ebola. Some As get copied over as Cs, and so on. Most of these mutations either have no effect or leave the copy unable to function, but once in a while they make the copy better suited to survive in its environment than the original that produced it. This is the evolutionary process that leads to COVID variants. Figuring out the order in which an organism's genetic letters appear is what's known as sequencing its genome. By sequencing the genomes of many different versions of a virus and studying the different mutations among them, scientists can construct what amounts to its family tree. At the bottom of the tree is the latest generation. Further up the tree are that generation's ancestors, all the way up to the first known specimen. The places where the tree branches split indicate major evolutionary steps, such as the emergence of a new variant, and the tree can even be used to record related pathogens that have been found in animals and might make the leap to humans. All this family tree information, in combination with a good testing regime, can provide invaluable insights into how a disease moves through a community. In South Africa, for instance, a good testing system paired with genetic analysis of HIV revealed that many young women who were living with the virus had acquired it by having sex with older men-- information that led to changes in the way the country approached HIV prevention. More recently, genetic sequencing revealed that a 2021 outbreak of Ebola in Guinea started with a nurse who had been infected, astonishingly, five years earlier. Scientists were stunned to learn that the virus could remain dormant for so long, and based on this new information, many are now rethinking ways to prevent Ebola outbreaks.