**Question:** 2009 Swine flu is due to:

A. H1N1 influenza A virus
B. H3N2 influenza A virus
C. Influenza B virus
D. Parainfluenza virus

**Core Concept:**
Swine flu, also known as pandemic (H1N1) 2009 influenza, is a respiratory illness caused by the swine influenza A virus, which is also known as the H1N1 virus. These viruses are related to those found in pigs but have been modified through reassortment with other influenza viruses, leading to the emergence of the new strain in humans.

**Why the Correct Answer is Right:**
The correct answer, A, indicates that the 2009 swine flu is caused by the H1N1 influenza A virus. This virus is a reassortant strain, meaning it has genetic material from both avian (bird) and human influenza viruses. The H1N1 virus was initially found in pigs and can cause swine influenza, but it has also adapted to infect humans, leading to the 2009 pandemic.

**Why Each Wrong Option is Incorrect:**

B (H3N2 virus): This option refers to the H3N2 strain of the influenza A virus, which is responsible for seasonal influenza in humans. It is not related to the 2009 swine flu pandemic.

C (Influenza B virus): Influenza B viruses are another strain of the influenza virus but are not responsible for swine flu. They cause seasonal influenza in humans but not the 2009 pandemic.

D (Parainfluenza virus): Parainfluenza viruses are responsible for respiratory tract infections, mainly in children, but they are not related to swine influenza or the 2009 pandemic.

**Clinical Pearl:**
Understanding the origin and transmission of the H1N1 virus is crucial for healthcare professionals to recognize and manage swine flu cases appropriately. In summary, the correct answer (A) highlights the importance of pandemic preparedness and the role reassortment plays in the emergence of new strains of influenza viruses.