Clusters of childhood leukemia, rare cancers, eye cancer, and multiple sclerosis have been reported in close-knit and isolated communities. When immortal pathogens are introduced into a close-knit or isolated community, the pathogens can be transmitted by people, animals, and birds, causing infection, re-infection, and chronic disease. Over time, chronic infections and re-infections can develop into a cluster of chronic disease. Chronic disease also has a genetic cost; and even a small genetic cost can devastate a family, group, or community, over many generations.