President Obama, terming the outbreak “cause for deep concern but not panic,” took the unusual step of using a prime-time televised news conference, convened to mark his 100th day in office, to deliver a public health message to the American people.

Image Commuters wore protective masks at a subway station in Mexico City on Wednesday. Credit... Daniel Aguilar/Reuters

“Wash your hands when you shake hands, cover your mouth when you cough,” he said from the East Room of the White House. “It sounds trivial, but it makes a huge difference. If you are sick, stay home. If your child is sick, take them out of school. If you are feeling certain flu symptoms, don’t get on an airplane.”

With public health officials recommending that schools close if there are more confirmed or suspected cases, Mr. Obama urged parents and businesses to “think about contingency plans” in case of such closings.

He said he was calling on Congress to authorize an immediate $1.5 billion to “support our ability to monitor and track this virus” and to build the supply of antiviral drugs.

“The more recent illnesses and the reported death suggest that a pattern of more severe illness associated with this virus may be emerging in the U.S.,” the C.D.C. said on its Web site. More hospitalizations and deaths are expected, the site said, because the virus is new and most people have no immunity to it.

Dr. Chan said that government preparedness plans could include steps like ensuring that laboratories can test for the disease and that health systems can identify and treat cases, track an outbreak and prevent the virus from spreading in hospitals and clinics. She said governments should also decide on measures like closing schools and discouraging or banning public gatherings.

Mexico, for instance, has prohibited people from eating in restaurants and ordered most stores and other businesses to close for several days starting Friday, a move apparently intended to keep people at home during what is traditionally a long holiday weekend.