IN RECENT WEEKScovid-19, a deadly new disease, has slowed in China but spread widely elsewhere. China's strict quarantine has led to a 90% decline in new infections, whereas outbreaks in Italy and Iran have grown rapidly. In the last week of February 70% of new diagnoses were outside of China. As covid-19 reaches countries unwilling or unable to monitor it, officials must use educated guesswork to track its evolution.
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