Sentence:During the first documented outbreak of human infections with H5N1, which occurred in Hong Kong in 1997, the 18 human cases coincided with an outbreak of highly pathogenic avian [e1]influenza[/e1], caused by a virtually identical [e2]virus[/e2], in poultry farms and live markets.
 e1: influenza. 
 e2 : virus.
Answer: Cause-Effect(e2,e1)