It’s one of the best weapons we have to contain a pandemic. But can it defeat the disease without spying on people who might carry it? MIT’s Kevin Esvelt has a bold idea: Let’s try a new form of contact tracing that could more than double the program’s impact. Bi-directional tracing looks both forward and backward from a known transmission, building a chart of the “undiscovered branches of the viral family tree,” and identifying potential spreaders other systems can’t see. But how much of our data are we willing to give the government, even if it’s to fight Covid-19?