Our slowest defense response is through natural selection, which changes our genotype frequency from generation to generation. For most illnesses, some people tend to be more resistant to genetics than others. Don’t tell me what to do cat poster. When an outbreak occurs, people with genes resistant to the same germ are more likely to survive than those without the gene. As a result, over the course of history, human populations regularly exposed to a particular pathogen eventually came up with a high proportion of individuals carrying genes that are resistant to that disease, simply because those individuals unfortunately do not have that gene, it is difficult to survive to pass on their genes to their children.