This is the thrilling, disruptive potential of "mHealth," the rapidly growing business of using mobile technology in health care. Leveraging the wonders of a device that's fast becoming ubiquitous--two in three people worldwide own a cell phone--a new generation of startups is building apps and add-ons that make your handheld work like high-end medical equipment. Except it's cheaper, sleeker, and a lot more versatile. "It's like the human body has developed a new organ," says Raja Rajamannar, chief innovation officer at Humana. Smartphones can already track calories burned and miles run, and measure sleep patterns. By 2013, they'll be detecting erratic heartbeats, monitoring tremors from Parkinson's disease, and even alerting you when it's prime time to make a baby.