As people would have no naturalimmunity, 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。
2
But knowing how much naturalimmunity 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。
3
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.