This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
The 1997 bird flu pandemic was caused by which of the following strain? 
Select and explain. 
A. H1N1, B. H5N1, C. H2N1, D. H4N1
B: H5N1
Answer: B: H5N1 
Explanation: Human infections with an influenza H5N1 was first repoed in Hong Kong in 1997. Ref: Epidemiology of communicable diseases Park's textbook of Preventive and Social Medicine, 21stedition, page-330.