The American testing supply chain is stretched to the limit, and the ongoing outbreak in the South and West could overwhelm it, according to epidemiologists and testing-company executives. While the country’s laboratories have added tremendous capacity in the past few months—the U.S. now tests about 550,000 people each day, a fivefold increase from early April—demand for viral tests is again outpacing supply.

If demand continues to accelerate and shortages are not resolved, then turnaround times for test results will rise, tests will effectively be rationed, and the number of infections that are never counted in official statistics will grow. Any plan to contain the virus will depend on fast and accurate testing, which can identify newly infectious people before they set off new outbreaks. Without it, the U.S. is in the dark.

The delays have already started. Yesterday, Quest Diagnostics, one of the country’s largest medical-testing companies, said that its systems were overwhelmed and that it would now be able to deliver COVID-19 test results in one day only for hospitalized patients, patients facing emergency surgery, and symptomatic health-care workers. Everyone else now must wait three to five days for a test result.