Emerging data as well as knowledge from the SARS and MERS coronavirus outbreaks yield some clues as to why SARS-CoV-2 affects some people worse than others.

Like many other respiratory conditions, COVID-19—the disease caused by SARS-CoV-2—can vary widely among patients. The vast majority of confirmed cases are considered mild, involving mostly cold-like symptoms to mild pneumonia, according to the latest and largest set of data on the new coronavirus outbreak released February 17 by the Chinese Center for Disease Control and Prevention.

Fourteen percent of confirmed cases have been “severe,” involving serious pneumonia and shortness of breath. Another 5 percent of patients confirmed to have the disease developed respiratory failure, septic shock, and/or multi-organ failure—what the agency calls “critical cases” potentially resulting in death. Roughly 2.3 percent of confirmed cases did result in death.

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