**Core Concept**
Swine flu, also known as pandemic influenza, is a respiratory disease caused by the influenza A virus, specifically the H1N1 subtype. This subtype is responsible for the 2009 global pandemic and is one of the most common causes of swine flu.

**Why the Correct Answer is Right**
The H1N1 subtype is a type of influenza A virus that primarily infects pigs but can also be transmitted to humans. The 2009 H1N1 pandemic highlighted the potential for rapid global spread of this virus. The H1N1 subtype has a unique combination of surface proteins, hemagglutinin (H1) and neuraminidase (N1), which facilitate its transmission and virulence.

**Why Each Wrong Option is Incorrect**
* **Option B:** H5N1 is another subtype of the influenza A virus, commonly referred to as bird flu. It is primarily found in birds but can be transmitted to humans, causing severe respiratory disease. However, it is not the primary cause of swine flu.
* **Option C:** H3N2 is another subtype of the influenza A virus, responsible for seasonal outbreaks in humans. While it can cause respiratory illness, it is not associated with swine flu.
* **Option D:** B virus refers to the influenza B virus, which is a separate type of influenza virus that causes seasonal outbreaks in humans. It is not associated with swine flu.

**Clinical Pearl / High-Yield Fact**
The 2009 H1N1 pandemic highlights the importance of global surveillance and rapid response to emerging infectious diseases. This incident led to significant changes in public health preparedness and response strategies worldwide.

**Correct Answer:**
✓ Correct Answer: A. H1N1