Organisms dangerous to people and animals continue to mutate, adapt, and spread, and opportunities for new diseases to emerge and travel globally are increasing.

对人和动物存在威胁的有机体扔在持续变异，适应环境并不断扩散；新疾病出现和全球传播的概率也在不断增加。

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When the flu spreads person to person, instead of from animals to humans, it can continue to mutate, making it harder to treat or fight, because people have no natural immunity.

CNN: WHO raises pandemic alert to second-highest level

When the flu spreads person to person, instead of from animals to humans, it can continue to mutate, making it a tougher strain that is harder to treat or fight off.

CNN: U.S. prepares for possible swine flu epidemic as global cases rise