On Sunday, Pope Francis condemned racism, comparing it to a virus that lurks in the shadows, only to erupt and demonstrate that “our perceived social progress” is “not as true or definitive” as people believe.

On the United Nations’ International Day for the Elimination of Racial Discrimination, Francis tweeted about racism.

Racism, according to the pope, is like “a virus that easily mutates and, instead of vanishing, goes into hiding and lurks in waiting.”

“Instances of racism continue to shame us, because they demonstrate that our ostensible social progress is not as true or definitive as we believe,” Francis wrote on Twitter, using the hashtags #FightRacism and #FratelliTutti. The title of the encyclical, or special teaching paper, issued by Pope Francis last year in the midst of the COVID-19 pandemic to call for global unity, brotherhood, and environmental security is “Fratelli Tutti.”

Francis did not mention any specific instance of racism or location in his tweet. Throughout his pontificate, he has spoken for the interests of those who are socially disadvantaged, such as refugees.

The annual United Nations commemoration on March 21 commemorates the day in 1960 when police in Sharpeville, South Africa, opened fire on a peaceful protest against apartheid laws, killing 69 protesters.