In part that's because occasionally the flu will change radically in what is called a shiftand take humans by such complete surprise that a pandemic erupts.
In part that's because occasionally the flu will change radically in what is called a shiftand take humans by such complete surprise that a pandemic erupts.
部分原因是流感病毒有时候会完全变了一个样，称为「大变异」，让人类惊觉流感已爆发大流行。