**Core Concept:** In epidemiological investigations, the search for cases is usually conducted until the source of infection is identified or the active transmission stops.

**Why the Correct Answer is Right:** In epidemics, the primary goal of case finding is to determine the source of the infection and interrupt the transmission chain. This is typically done by continuous case identification until the source (index case) is found, or the period of infectiousness for the remaining cases is determined, or the epidemic curve flattens (indicating successful control measures).

**Why Each Wrong Option is Incorrect:**

A. **Continuous:** This option is incorrect because it does not reflect the practicality of the investigation process. In reality, the search for cases will eventually taper off once the source is identified or transmission ceases.

B. **Until all cases are found:** This option is also incorrect, as finding all cases in an epidemic is often not feasible due to limitations such as limited resources, time, and accessibility of individuals.

C. **Until case-control study:** A case-control study is a research design used in epidemiology to identify risk factors associated with a disease or condition. This option is unrelated to the duration of case finding in an epidemic.

D. **Until zero cases are found:** This option is also incorrect because it is unrealistic and does not consider the practical challenges in finding all cases.

**Clinical Pearl:** In practice, the search for cases in an epidemic should be guided by the goal of identifying the source of infection and interrupting transmission, rather than aiming for a specific number of cases or time period. This approach ensures efficient use of resources and better control of the epidemic.

**Correct Answer:** **Until the source is identified or transmission stops.**