Letting Covid-19 circulate in hope of herd immunity 'could make it more lethal'

Study says efforts to prevent spread of disease, such as social distancing, reduce virulence

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A deeply entrenched idea, that newly emerged agents of disease inevitably evolve to become more benign over time, is scientifically unfounded, according to new research.

They can, in fact, become more virulent depending on the conditions, and the easier it is for a virus to spread, the more likely it is to do so.