**Core Concept**
Isolation as a method of control is primarily used for diseases that are highly contagious and easily transmissible from person to person, such as influenza or tuberculosis. The goal of isolation is to prevent the spread of the disease to others. However, in some cases, isolation may not be the most effective method of control.

**Why the Correct Answer is Right**
The correct answer is **Diphtheria**. Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae. While it can be transmitted from person to person, it is not as contagious as some other diseases, and isolation may not be the most effective method of control. This is because the bacteria can also be spread through contaminated food and water, and through contact with infected animals.

**Why Each Wrong Option is Incorrect**
**Option A:** **Influenza** - Influenza is a highly contagious respiratory illness that spreads easily from person to person, making isolation a crucial method of control.
**Option B:** **Tuberculosis** - Tuberculosis is a highly contagious bacterial infection that can spread through the air when an infected person coughs or sneezes, making isolation essential.
**Option C:** **Measles** - Measles is a highly contagious viral infection that spreads easily from person to person through the air, making isolation a critical method of control.

**Clinical Pearl / High-Yield Fact**
Diphtheria is a vaccine-preventable disease, and widespread vaccination has led to a significant decline in cases. However, the disease still exists in some parts of the world, and healthcare workers should be aware of its transmission dynamics.

**Correct Answer:** D. Diphtheria