Historical epidemiology has taken this latter-day awareness and extended it to explain the dynamics of infectious and contagious diseases, many of which originated in animal herds and then adapted to human hosts. Within an individual society, pathogens such as the viruses that cause smallpox and measles can become endemic if the population is large enough to enable the viruses to find constant supplies of fresh hosts who do not possess inherited or acquired immunities. In those cases, the diseases become childhood diseases that exact a regular but limited toll in human lives. But if a society has insufficient population to support the diseases on an endemic basis, or if a society has never been exposed to a given pathogen, then few if any individuals will possess immunities. If a particularly virulent pathogen like the smallpox virus should find its way into such a society, it can touch off massive and destructive epidemics.