The H5N1 bird flu virus first appeared in 2003, it has killed more than half of the 622 people it has infected. WHO and Chinese officials say the effort to understand the new virus is still early.H5N1型禽流感最早出现于2003年,造成622名感染者中超过半数人死亡.世卫组织和中国官员表示,对这种新型病毒的研究仍处于早期阶段.
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