In the coronavirus pandemic, the next testing challenge will be to diagnose the recovered.
Identifying those who are no longer sick with COVID-19 may not seem as urgent as testing those who may be newly infected. So why scramble to confirm that patients have returned to health or find ones who never seemed sick in the first place?
The answer: Because they can provide care, and perhaps a cure, for those who are sick with COVID-19. And their numbers will offer a barometer of how near we are to this pandemic's end.
Public health authorities suspect that tens of thousands of Americans, if not more, have had COVID-19 without realizing it because they suffered few or very mild symptoms.
Around the world, more than 100,000 former patients have been tagged as "recovered." But that is likely a tiny fraction of the true total.
Now immune to reinfection -- at least for a while -- many of these recovered people could safely go back to work. They could care for those who are ill or who are at high risk of dying if they become ill. They could donate their antibody-rich blood to be used as medicine for the critically ill.
They could be among the first to return to life as we used to know it.
But first, officials need a test to identify this population.
With President Donald Trump holding out the imminent prospect of resuming our usual ways, health officials say that knowing who had already recovered, and what percentage of the population they represent, would be vital for a safe return to normal.
Understanding how many people were immune to the new coronavirus would help public health officials anticipate their communities' health care needs by assessing how many remain vulnerable and how aggressive the virus actually is.