As people would have no natural immunity, a new influenza virus could cause widespread death, illness, social and economic disruption.

由于人无天然的免疫力，一种新的流感病毒可引起广泛的死亡、疾病、社会和经济破坏。

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And smiling has been clinically shown to lower blood pressure, boost immunity, and even release natural painkillers (endorphins), each of which lowers stress, boosts happiness, and improves health.

微笑在临床上的表现能降低血压，提高免疫力，甚至还能释放出天然止痛药(内啡肽)，这每样都能减轻压力、增加快乐和促进健康。

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But knowing how much natural immunity has been acquired through infection is more difficult, especially given the very mild nature of illness in the vast majority of patients.

但很难说清因感染获得了何种程度的天然免疫力，尤其是因为在绝大多数患者身上，疾病的性质非常温和。

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When people from non-endemic areas, who have little natural immunity, travel to endemic ones, they are especially vulnerable.

ECONOMIST: Malaria: Four horsemen of the Apocalypse? | The

However, since natural immunity to malaria exists, researchers are reasonably optimistic that a vaccine, which stimulates this immunity artificially, should be feasible.

ECONOMIST: Malaria research gets a shot in the arm

Director of the Public Health Agency Dr Caroline Harper said this age group was vaccinated last year and now 50-60% had built up some level of natural immunity.

BBC: Health Committee