Dr. Anthony Fauci, the nation's leading infectious disease expert, has been working non-stop for months to fight the Covid-19 pandemic.

President-elect Joe Biden asked Fauci to serve as his Chief Medical Advisor in addition to his longtime role as the director of the National Institute of Allergy and Infectious Diseases on Thursday. "I said yes right on the spot," Fauci told NBC's "TODAY" on Friday.