**Core Concept**
Typhoid and cholera are examples of diseases where healthy carriers play a significant role in the transmission of the disease. Healthy carriers are individuals who harbor the pathogen in their bodies but do not exhibit symptoms. These carriers can potentially infect others through various means, such as fecal-oral transmission. In contrast, diphtheria does not typically involve healthy carriers, as the bacteria primarily colonize the respiratory tract and cause symptoms in the infected individual.

**Why the Correct Answer is Right**
Typhoid is caused by Salmonella Typhi, which can be harbored by healthy carriers, particularly in the gallbladder. These carriers can shed the bacteria in their stool, contaminating food and water, and thus contributing to the spread of the disease. In contrast, diphtheria is caused by Corynebacterium diphtheriae, which primarily colonizes the respiratory tract and does not have a significant reservoir in healthy carriers. The bacteria are usually acquired through respiratory droplets from an infected individual.

**Why Each Wrong Option is Incorrect**
**Option B:** Cholera is caused by Vibrio cholerae, which can be harbored by healthy carriers, particularly in the gastrointestinal tract. These carriers can shed the bacteria in their stool, contaminating food and water, and thus contributing to the spread of the disease.
**Option C:** While diphtheria does not typically involve healthy carriers, it is not entirely accurate to say that it does not involve carriers at all. However, the primary mode of transmission is through respiratory droplets from an infected individual, rather than through healthy carriers.
**Option D:** This option is incorrect because it states that all of the above diseases do not have healthy carriers, which is not true for typhoid and cholera.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the role of healthy carriers in the transmission of certain diseases, such as typhoid and cholera. This knowledge can inform public health strategies aimed at controlling the spread of these diseases.

**✓ Correct Answer: A. Typhoid**