**Core Concept**
Emergency or reemergence of infectious diseases can occur due to various factors, including changes in human behavior, climate, or the emergence of new viral strains. In the context of the question, we are looking for a virus that has experienced an increase in incidence or a return to a previously high level of transmission.

**Why the Correct Answer is Right**
The correct answer is **a) Polio virus**. Poliovirus, the causative agent of poliomyelitis, has experienced reemergence in several countries, including Afghanistan and Pakistan, due to factors such as inadequate vaccination coverage, conflict, and poor sanitation. The reemergence of poliovirus is a significant public health concern, as it can lead to severe and lifelong paralysis.

**Why Each Wrong Option is Incorrect**
**Option B:** Measles virus is still a major public health concern, but it is not considered to be in a state of reemergence. Efforts to control and eliminate measles have been successful in many parts of the world, although outbreaks still occur.

**Option C:** Nipah virus is a zoonotic virus that can cause severe respiratory and neurological disease in humans. While it is a significant public health concern, it is not typically associated with reemergence in the same way that poliovirus is.

**Option D:** West Nile virus is a flavivirus that can cause severe neurological disease in humans, but it is not typically associated with reemergence in the same way that poliovirus is.

**Option E:** Hepatitis B virus is a major public health concern worldwide, but it is not typically associated with reemergence in the same way that poliovirus is.

**Clinical Pearl / High-Yield Fact**
Poliovirus reemergence is often linked to inadequate vaccination coverage, particularly in areas with low immunization rates. Healthcare providers should be aware of the risk factors for poliovirus transmission and ensure that patients are up-to-date on their polio vaccinations.

**Correct Answer: A. Polio virus**