The coronavirus pandemic has sickened more than 522,800 people, according to official counts. As of Friday morning, at least 23,895 people have died, and the virus has been detected in at least 171 countries, as these maps show.

There is evidence on six continents of sustained transmission of the virus. The C.D.C. has advised against all non-essential travel throughout most of Europe, and to South KoreaChina and Iran. And the agency has warned older and at-risk Americans to avoid travel to any country.