Dr. Anthony Fauci has announced that in December he will step down from his positions as chief medical adviser to President Joe Biden and as head of the National Institute of Allergy and Infectious Diseases and its laboratory of immunoregulation.

Fauci has worked for the National Institutes of Health since 1968 and has been the director of NIAID since 1984. In that time, he has advised seven U.S. presidents on infectious disease threats such as HIV and AIDS; the West Nile, Ebola and Zika viruses; and more.

But most of the public may know him as the face of the federal response to the coronavirus pandemic, first as a member of former President Donald Trump’s White House Coronavirus Task Force and now as a member of Biden’s response team. Not everyone has been a fan.

Republican politicians and conservative media outlets have continually criticized his efforts to slow the spread of COVID-19, questioned his motivations for promoting vaccination against it, and speculated on what he knows about the origin of the virus. GOP members of Congress have promised to investigate Fauci and have him testify before Congress if Republicans retake control of the House or Senate next year.