Influenza pandemic is due to
**Core Concept**
Influenza pandemic is caused by the emergence of a new **influenza A virus** subtype, which has the ability to infect humans and spread efficiently from person to person. This occurs due to **antigenic shift**, a process where there is a major change in the viral genome.
**Why the Correct Answer is Right**
The correct answer is related to the **emergence of a new influenza A virus subtype**. This emergence is often due to **reassortment** of viral genes from different influenza viruses, typically from animal sources such as birds or pigs, resulting in a new virus strain to which the human population has little or no immunity.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the mechanism or type of virus involved.
**Option B:** Incorrect as it may refer to seasonal influenza, not pandemic.
**Option C:** Incorrect because it doesn't account for the specific viral subtype or antigenic shift.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **influenza pandemics** are characterized by widespread disease, high morbidity, and significant mortality, often due to the lack of immunity in the population against the new viral strain.
**Correct Answer:** D. Antigenic shift due to emergence of a new influenza A virus subtype.