Dr. Thomas Frieden, who runs the U.S. Centers for Disease Control and Prevention, learned Thursday what happens when a crucial American health sentry is caught snoozing during an invasion.

The agency charged with keeping America safe from life-threatening infectious diseases was forced to face hard questions from a congressional panel -- and to admit that it didn't know how two nurses contracted the Ebola virus from patient Thomas Eric Duncan in Dallas this month.

But the CDC has dramatically tightened its safety protocols, it told the committee hopefully. That's good, but it's a little late when the agency is trying to convince a nation to stay calm as details about the horrible pathogen unfold.

The agency's new rules are similar to those of Doctors Without Borders, a nonprofit that works closely with the CDC in Africa and has employed tight procedures. That includes full bodysuits and careful supervision in the treacherous process of removing all the protective layers.

But why did the CDC take these measures only after fears of contagion arose? Texas Health Presbyterian Hospital in Dallas said nurse Nina Pham contracted the Ebola virus after treating Duncan. Another one of Duncan's nurses, Amber Joy Vinson, flew to Cleveland even though she was being monitored. And was allowed to return to Dallas after she reported a fever. Now, she has the virus.

Of course, fear is spreading: 70 percent of Americans said in a poll that they're closely following news of Ebola. Schools in Texas and Ohio with students or employees on one of those flights were closing Thursday. To calm concerns in New York, Gov. Andrew Cuomo announced a readiness plan that includes eight "super centers" for Ebola patients -- four in NYC. Cuomo said he "wouldn't be surprised" if a case arrived here.

Frieden told the congressional committee that a travel ban between West Africa and the United States doesn't make sense because people would find other ways to get here. But he added such a measure hadn't been ruled out. And as pressure mounts to reassure the nation, the White House may have no choice but to impose a ban.

This is what happens when slipshod work leads to a significant erosion of trust.