**Core Concept**
The index case is a crucial concept in epidemiology, referring to the first individual whose disease or infection is recognized by investigators or healthcare professionals, often triggering the investigation of an outbreak or epidemic. This individual serves as a sentinel, helping to identify potential sources of infection and guiding further investigation.

**Why the Correct Answer is Right**
The correct answer, **D. First case detected by investigator**, highlights the significance of the index case in initiating an investigation. When a healthcare worker or investigator identifies an individual with a disease or infection, it marks the beginning of an epidemiological investigation. This initial case helps to trigger further inquiry into the source of the infection, potential transmission routes, and the scope of the outbreak. The index case is often used as a reference point to calculate the incubation period, secondary attack rate, and other key epidemiological metrics.

**Why Each Wrong Option is Incorrect**
**Option A:** The first case in a community is not necessarily the index case, as it may not have been identified or reported by investigators.
**Option B:** The case getting infection from a primary case is actually the secondary case, not the index case.
**Option C:** Similarly, the case getting infection from a secondary case is a tertiary case, not the index case.

**Clinical Pearl / High-Yield Fact**
A key takeaway from this concept is that the index case is not necessarily the first person to become infected with a disease, but rather the first individual whose infection is recognized and reported by investigators. This distinction is crucial in understanding the epidemiological dynamics of outbreaks and in guiding public health interventions.

**✓ Correct Answer: D. First case detected by investigator**