## Core Concept
The International Health Regulations (IHR) are an international legal instrument that is binding on 196 countries across the globe, including all the Member States of WHO. The purpose of IHR is to prevent, protect against, control, and provide a public health response to the international spread of disease in ways that are commensurate with and restricted to public health risks, and which avoid unnecessary interference with international traffic and trade. The IHR (2005) introduced the concept of "public health emergency of international concern" (PHEIC) and requires countries to report specific diseases.

## Why the Correct Answer is Right
The correct answer, **C. Hepatitis B**, is not included in the list of diseases that are internationally notifiable under the older versions of the International Health Regulations (IHR). The classic notifiable diseases include Cholera, Plague, and Yellow Fever. These diseases have been historically significant in terms of international spread and public health impact. Hepatitis B, while a significant public health concern, is not among the traditionally notifiable diseases under older IHR versions but is subject to reporting under current public health surveillance.

## Why Each Wrong Option is Incorrect
* **Option A: Cholera** is indeed one of the diseases that have been historically notifiable internationally due to its potential for rapid spread and high mortality if not treated promptly.
* **Option B: Plague** is another disease that has been internationally notifiable because of its potential to cause significant morbidity and mortality and to spread rapidly across international borders.
* **Option D: Yellow Fever** is also internationally notifiable due to its high mortality rate and potential for spread, particularly in areas where its vector, the Aedes mosquito, is prevalent.

## Clinical Pearl / High-Yield Fact
A key point to remember is that under the current IHR (2005), countries are required to report any event that could constitute a public health emergency of international concern (PHEIC), regardless of the specific disease. This represents a shift from the older system, which specified certain diseases.

## Correct Answer: C. Hepatitis B