This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
Swine flu is caused most commonly by - 
Select and explain. 
A. H1N1, B. H5N1, C. H3N2, D. B virus
A: H1N1
Answer: A: H1N1 
Explanation: <p> SWINE FLU:- Pandemic influenza A (H1N1). This differs from seasonal influenza in two ways. 1. Majority of population has little/no pre existing immunity to virus,the impact of the infection thus leading to wider range. 2. The virus infect the lower respiratory tract and can cause progressive pneumonia. Incubation period:- 2-3 days but can range upto 7 days. {Reference: park&;s textbook of preventive and social medicine, 23rd edition, pg no.156}</p>