Early mistakes in testing, aging data systems, clashes with President Trump and an overly cautious culture shook confidence in the nation’s premier public health agency.

WASHINGTON — Long considered the world’s premier public health agency, the Centers for Disease Control and Prevention has fallen short in its response to the most urgent public health emergency in its 74-year history — a pathogen that has penetrated much of the nation, killing more than 100,000 people.

But a New York Times review of thousands of emails, and interviews with more than 100 state and federal officials, public health experts, C.D.C.

As the virus began to spread in the United States in January, the C.D.C.’s response was hampered by an antiquated data system and a fractured public health reporting system across the U.S., relying in part on information assembled or shared with local health officials through phone calls, faxes and thousands of spreadsheets attached to emails.

The C.D.C.

The overall result was an agency that had blind spots at just the wrong moment.

and state and local public health departments delayed sharing critical information that could help patients.

“We got crappy data,” said Fran Phillips, Maryland’s deputy health secretary.

He said the virus would disappear “like a miracle” even as C.D.C.

urged Americans to wear masks, he said, “I don’t see it for myself.”.

to coordinate with the White House,” said Lawrence Gostin, the director of a legal center at the World Health Organization and a former C.D.C.

The culture at the C.D.C.

Mass protests against police brutality that have brought thousands of people onto the streets in cities across America are raising the specter of new coronavirus outbreaks, prompting political leaders, physicians and public health experts to warn that the crowds could cause a surge in cases.

While many political leaders affirmed the right of protesters to express themselves, they urged the demonstrators to wear face masks and maintain social distancing, both to protect themselves and to prevent further community spread of the virus.

A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals.

The best way to protect yourself from coronavirus — whether it’s surface transmission or close human contact — is still social distancing, washing your hands, not touching your face and wearing masks.

The C.D.C.

has recommended that all Americans wear cloth masks if they go out in public.

This is a shift in federal guidance reflecting new concerns that the coronavirus is being spread by infected people who have no symptoms.

Until now, the C.D.C., like the W.H.O., has advised that ordinary people don’t need to wear masks unless they are sick and coughing.

Part of the reason was to preserve medical-grade masks for health care workers who desperately need them at a time when they are in continuously short supply.

If you’re sick and you think you’ve been exposed to the new coronavirus, the C.D.C.

The coronavirus pandemic underscored the need for Dr.

As the national clearinghouse for critical public health information, the C.D.C

But the agency has struggled at times to provide clear and timely guidance, leading many to say they now look to universities, mailing lists or online research articles for detailed recommendations about how to safely care for infected patients

After initially recommending that all doctors and nurses coming in contact with coronavirus patients wear N95 respirators, the C.D.C

The change angered health care workers like Lori Freeman, chief executive of the National Association of County and City Health Officials, who said that “mistrust crept in.”

As Miami Beach begins to reopen its economy, Mayor Dan Gelber said he wished the C.D.C

An agency press officer said the C.D.C