A mysterious piece of malware has been infecting hundreds of Mac computers for years—and no one noticed until a few months ago.

Earlier this year, an ex-NSA hacker started looking into a piece of malware he described to me as "unique" and "intriguing." It was a slightly different strain of a malware discovered on four computers earlier this year by security firm Malwarebytes, known as "FruitFly." This first strain had researchers scratching their heads. On the surface, the malware seemed "simplistic." It was programmed mainly to surreptitiously monitor victims through their webcams, capture their screens, and log keystrokes.