Dec. 31
The World Health Organization says that Chinese health officials in Wuhan revealed a “cluster of cases of pneumonia of unknown cause.”
Jan. 8
The C.D.C. confirms the first travel-related case of coronavirus in the United States: 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.”
Jan. 31
The Trump administration restricts travel from China, but exempts Americans and allows trade to continue. Mr. Azar declares a public health emergency.
Early February
Stanford University develops its own test for the coronavirus but runs into regulatory roadblocks at the F.D.A.
Feb. 4
The F.D.A. approves the C.D.C. test, but the White House task force remains focused on the Diamond Princess cruise ship, which is placed under quarantine in Japan because 10 people on board are diagnosed with the coronavirus.
Feb. 12
The C.D.C. publicly discloses that its test kits aren’t working properly amid complaints from labs around the country that screening has been far too restricted.
Feb. 14
The C.D.C. announces that it will begin surveillance testing to track the virus. But a month later, it says it hopes to “begin rolling out” the program, which has yet to begin in a significant way.
Feb. 24
The chief executive of the Association of Public Health Laboratories writes to the F.D.A. that “we are now many weeks into the response with still no diagnostic or surveillance test available outside of C.D.C. for the vast majority of our member laboratories.” Separately, the head of the C.D.C. boasts to Congress about his agency’s “aggressive response” to screening.
Feb. 29
The F.D.A. relaxes its rules for some laboratories, allowing them to start testing before the agency has completed its approvals. At a White House news conference, Mr. Trump concedes there will be more cases, but says, “There’s no reason to panic at all.”
March 1
New York City reports its first case.