The new coronavirus could be a “once-in-a-century pathogen we’ve been worried about,” according to Microsoft co-founder Bill Gates.

“I hope it’s not that bad, but we should assume it will be until we know otherwise,” the billionaire philanthropist said in a Friday article for the New England Journal of Medicine. “There are two reasons that Covid-19 is such a threat. First, it can kill healthy adults in addition to elderly people with existing health problems. The data so far suggest that the virus has a case fatality risk around 1%; this rate would make it many times more severe than typical seasonal influenza, putting it somewhere between the 1957 influenza pandemic (0.6%) and the 1918 influenza pandemic (2%).”