In a contagious manner; in a way that spreads or transmits disease or influence readily from one person to another.
From 'contagious' (adjective) plus '-ly' (adverb suffix). This adverbial form emphasizes the manner or degree to which something transmits or spreads.
When people say someone smiled 'contagiously,' they're using a disease metaphor poetically—the metaphor is so natural that we forget we're comparing emotion to plague, showing how deeply these concepts intertwine in our thinking.
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