Dec. 31

The World Health Organization says that Chinese health officials in Wuhan revealed a “cluster of cases of pneumonia of unknown cause.”

Jan. 20

The United States has its first confirmed case of coronavirus through travel: a man in Washington State.

Jan. 24

The C.D.C. says that it has developed a sophisticated diagnostic test and has sought F.D.A. permission to send it to public labs around the country.

Jan. 29

The White House announces a coronavirus task force led by the health secretary, Alex M. Azar II. President Trump attends the group’s first meeting and tweets that the experts “are on top of it.”