How likely are you to answer an unknown number popping up on your caller ID? And if you were to pick up, how likely is it that you would disclose personal information about yourself and your family, friends and colleagues to the stranger on the other end of the line?

Ask any census worker, public opinion pollster or cold caller, and they will tell you the answer to both questions is “not likely.” Indeed, the pickup rate for unfamiliar or unidentified numbers in those circumstances is as low as 6 percent. And the people who do answer are often wary and uncooperative, if not abusive.

That resistance is the crux of the problem with contact tracing, which health experts say is essential to containing the coronavirus.