**Core Concept**
International Health Regulations (IHR) are a set of rules developed by the World Health Organization (WHO) to help countries work together to protect public health, prevent the spread of diseases, and control outbreaks. One crucial aspect of IHR is case notification, which involves reporting specific diseases to the WHO to facilitate global surveillance and response.

**Why the Correct Answer is Right**
Yellow fever is a viral disease transmitted by mosquitoes, primarily in Africa and South America. It is a significant public health concern in these regions, and its spread can be rapidly contained through early detection and response. According to IHR, yellow fever is one of the diseases that require immediate notification to the WHO, as it has the potential to cause large-scale epidemics and spread across international borders. This notification allows for rapid deployment of resources, coordination of response efforts, and implementation of control measures to prevent further transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Hepatitis B is a viral liver infection, but it is not typically considered a high-priority disease for IHR case notification, as it is not usually associated with rapid spread or severe public health consequences.
**Option B:** HIV is a significant global health concern, but its transmission is primarily through human-to-human contact, and it is not typically reportable under IHR due to its established presence in many countries.
**Option D:** This option is incorrect because only yellow fever is specifically mentioned in IHR as a disease requiring immediate notification.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that IHR case notification is not limited to yellow fever alone; other diseases like polio, SARS, and pandemic influenza also require immediate reporting. However, yellow fever is a notable example of a disease that triggers a rapid response under IHR.

**✓ Correct Answer: C. Yellow fever**