What is a story's atmosphere?

a. the use of music and lighting to make a setting seem more appealing
b. the air quality and temperature in the setting of a short story or poem
c. the mood or feeling of a story, usually established through details of setting
d. the most important element in a story, prioritized over plot and character

C. The mood or feeling of a story, usually established through details of setting

The answer is installation.

The constant presence of diseases or infectious agents within a given geographic area or population group. it may also refer to the usual prevalence of a given disease with such area or group. it includes holoendemic and hyperendemic diseases. a holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). a hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. so probably family history.