Final answer:
The term for the total number of existing cases of a disease in a given population is 'prevalence', which includes both new and old cases and serves as a snapshot of the disease's overall impact.
Step-by-step explanation:
The term that refers to the total number of existing cases (old & new) of a disease in a given population, usually expressed as a percentage, and is a "snapshot" to help assess the overall impact of a disease is prevalence. Prevalence estimates the likelihood that someone in a group will have a disease and is often used as an indicator of the overall healthcare burden of a disease. Unlike incidence, which measures the number of new cases during a specific time period, prevalence includes both new and existing cases, providing a measure of the total disease burden at a particular point in time.