Why does genetic variation increase the chance that some individuals in a population will survive?

Natural selection, an animal with white fur won't survive in an area with green grass and leaves, but a brown or a green animal would

Take this example. If a population is sole comprised of one species of one type, and a disease comes along that wipes out this population, then everyone dies. However, if there is some variation, or resistance to this disease, then the resistant population will survive. This means that the species lives on!