The state of Ohio has, as of this writing, five confirmed cases of Covid-19, the disease caused by the coronavirus SARS-CoV-2. If you ask Amy Acton, the director of the Ohio Department of Health, that’s the tip of the tip of a very dangerous iceberg. “We know now, just the fact of community spread says that at least 1 percent, at the very least 1 percent, of our population is carrying this virus in Ohio today," Acton said at a press conference on Thursday. "We have 11.7 million people." In other words, Acton was implying: 117,000 cases in Ohio alone.

How could Acton know this? Well, technically, she can’t. She was extrapolating from what little data scientists actually have about the spread of the pandemic. Officially, there have been just shy of 130,000 cases of Covid-19 on Earth, so a six-digit toll among Ohioans would be terrible, with extraordinary implications for how many cases might be out there undetected in the rest of the country.

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Whether that number is accurate or not is exactly the sort of thing public health workers would like to know. Who’s sick? How bad? How fast will the disease spread? But in the United States, nobody has good answers, for one simple but terrifying reason: There aren’t enough tests. Unlike for the flu, or previous coronaviruses like SARS or MERS, or sexually transmitted diseases, or a host of other infections, health workers have no way to find out whether a person sitting in front of them has Covid-19 or not. That technology actually exists and is relatively simple. They just don’t have access to it. Right now, four months into a global pandemic, two weeks since the first case of “community spread” within US borders, doctors and health workers have no way of testing everyone to see whether they have Covid-19. (That may at last be about to change. On March 13, the FDA granted approval for commercial tests from two companies, Swiss pharmaceutical giant Roche and medical device-maker Thermo Fisher. Their expertise in fast, bulk-scale medical test construction and distribution means their promise to roll out 2 million tests almost immediately is actually plausible.)

The American Enterprise Institute’s program to estimate total US testing capacity currently puts that number at just over 22,000 tests a day—roughly twice the daily capacity of South Korea, a country with just one-sixth the population of the US and about one-hundredth the square mileage. That shortfall limits what scientists know about the disease—and therefore what they know about how to fight it. America is behind a fearsome curve, looking at a potential explosion of infection numbers. A large percentage of Americans will get sick enough to require hospitalization, and some of them are going to die. But nobody had enough tests in January or February to try to get ahead of that problem, and nobody has enough tests now to know how big a wave is yet to come. The reasons why are both scientific and political, and it’ll take deft science and politics to fix them.

Broadly, the response to an infectious disease outbreak has two phases: containment and mitigation. The containment phase happens early, when initial cases begin to appear. Public health workers tasked with surveillance do interviews to determine all the people an infected person might have come into contact with, to notify them and either isolate or treat them, in an effort to reduce further spread. But you can see where this is headed: Without the ability to test whether people are infected, health workers can’t effectively wall off these networks.