patient zero
US /ˌpeɪ.ʃənt ˈzɪr.oʊ/
UK /ˌpeɪ.ʃənt ˈzɪr.oʊ/
1.
the first person to be identified as carrying an infectious disease in an outbreak of a disease
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Scientists are still trying to identify patient zero in the recent epidemic.
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Understanding patient zero can help trace the spread of a virus.