Director of the National Institute of Allergy and Infectious Diseases also says millions of Americans will be infected by the virus

Lockdowns spread from China, to Europe, to US as center of pandemic's impact shifts; 15,000 have died worldwide

US president breaks with senior health official over effectiveness of medications in treating COVID-19, berates reporters; announces effective closure of Mexico border

US president holds press conference after speaking with heads of US grocery chains; top public health expert calls for aggressive measures such as a 14-day national shutdown