Humans are social beings, and they rely on each other‘s existence to progress and survive.
Diseases occur and exist in social networks of people rather than just individuals. In epidemiology, we, therefore, look at patterns of whole populations and groups rather than individuals to understand diseases.
The social networks within a given population can make it more or less susceptible to infectious diseases. Some social networks of a region might sustain stable levels of a pathogen and maintain endemic disease dynamics within themselves. Having that endemic baseline can help pathogens with the multi-host transmission in the ecological community.
It is also important to understand that populations do not exist in isolation, especially in today’s global village. Epidemiologists then need to understand diseases in a dynamic environment as opposed to static models.