**Core Concept**
The question tests the understanding of infectious disease epidemiology, specifically the concept of disease reservoirs. A reservoir is any person, animal, arthropod, plant, soil, or substance, or a location which can harbor or transmit an infectious agent.

**Why the Correct Answer is Right**
The correct answer is Measles, as **Measles** is a highly contagious viral disease that is primarily spread from person to person. The virus has no other significant reservoir apart from humans, making humans the sole reservoir of the disease. This is crucial for disease control and elimination efforts.

**Why Each Wrong Option is Incorrect**
**Option A:** Rabies has multiple animal reservoirs, including dogs, cats, and bats, which can transmit the virus to humans.
**Option C:** Typhoid has a reservoir that includes asymptomatic human carriers, but the bacteria can also be found in contaminated food and water.
**Option D:** Japanese B encephalitis is primarily transmitted through the bite of an infected mosquito, with pigs and birds serving as the main reservoir.

**Clinical Pearl / High-Yield Fact**
Remember that diseases with a single reservoir, like **Measles**, can potentially be eradicated with targeted public health interventions, such as vaccination campaigns.

**Correct Answer:** B. Measles