Microsoft co-founder and billionaire Bill Gates wrote in an article published Friday that coronavirus may be the “once-in-a-century pathogen we’ve been worried about.”

“I hope it’s not that bad, but we should assume it will be until we know otherwise,” Gates wrote in the article, which was featured in the New England Journal of Medicine.

Gates' nonprofit, The Bill and Melinda Gates Foundation, focuses on fighting extreme poverty and improving global health. It announced at the beginning of February that it was donating $100 million to help find treatments and expand testing on the virus, CNBC reports.

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Gates, in his article, said that what makes COVID-19, the official name of the disease, concerning is that it is more contagious than other deadly diseases.