在极少数情况下，禽流感病毒已跨越物种屏障，感染人类。
On rare occasions, avian influenza viruses have crossed the species barrier to infect humans.
禽流感病毒具有高度物种特异性，但偶尔也跨越物种屏障，感染人类。
Avian influenza viruses are highly species-specific, but have, on rare occasions, crossed the species barrier to infect humans.
在跨越物种屏障感染人类的若干禽流感病毒中，H5N1导致的人类重症病例和死亡数目最多。
Of the few avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of cases of severe disease and death in humans.
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