The short answer is a lack of preparation and poor execution by the federal government. The initial tests developed by the Centers for Disease Control and Prevention had a technical problem — and federal officials were then too slow to find alternatives.

In his Oval Office address last night, President Trump tried to blame Europe for the spread of the virus in the United States. But Europe isn’t the problem (and the fact that indexes tied to the future of the stock market began falling during his speech suggests investors were unnerved by what Trump was saying). A much bigger problem is the lack of testing in the United States.

As Vox’s Brian Resnick and Dylan Scott explain: “Accurate testing is critical to stopping an outbreak: When one person gets a confirmed diagnosis, they can be put in isolation where they won’t spread the disease further. Then their contacts can be identified and put into quarantine — so that they don’t spread the virus if they’ve become infected, too. That’s particularly important for a virus like this one, which seems able to spread before people show symptoms, or when their symptoms are mild.”

How exactly have American officials botched the tests?

After problems arose with the C.D.C.’s test, officials could have switched to using successful tests that other countries were already using. But the officials refused to do so, essentially because it would have required changing bureaucratic procedures.

The federal government could also have eased regulations on American hospitals and laboratories, to allow them to create and manufacture their own tests, as Melissa Miller of the University of North Carolina at Chapel Hill School of Medicine told The Washington Post. But federal officials did not do so for weeks. The Times’s Sheri Fink and Mike Baker reported this week about a Seattle lab with a promising test that was blocked by “existing regulations and red tape” while “other countries ramped up much earlier and faster.”