the Shanghai woman had fallen ill on her flight back to China. She had tested positive for SARS-COV2. But here was the puzzle: she had had no symptoms when she had met him; she had seemed perfectly well. She had only fallen ill two days later. In short, she had transmitted the virus to the man while being presymptomatic. No one could have told her, or the exposed man, that she had been a carrier of the virus. No isolation or quarantine based on symptoms could have stopped the virus. The mystery deepened when the man was tested. His symptoms had defervesced by then; he had returned to work,
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