Three times a week, Dr. Joseph Olson dons his white coat, but he doesn't leave his home in Storden, Minn. Instead, he fires up his tablet , peers into the camera and starts "seeing" patients via real-time video chat. On the other end of the Internet connection are people who have logged on for the virtual visit through their smartphones or tablets . One by one, Olson examines them — diagnosing mostly minor ailments such as sinus infections, skin rashes and sprains. Olson works for "Doctor on Demand," a mobile app that started four months ago and is now in 40 states, with 1,000 doctors on staff.