Increases and decreases in the frequency of disease and health conditions over a period of years or within each year refers to

Increases and decreases in the frequency of disease and health conditions over a period of years or within each year refers to epidemiology.

Explanation:

In health studies, epidemiology can be described as the pattern of distribution of a disease in a defined population. The outbreak of a disease, its surveillance, screening etc are all measured by epidemiologists.

The measure of probability of a condition to occur in the population is termed as incidence. Prevalence can be described as the measure of population being affected by a circumstance.