At Doctor on Demand, a popular online health site, patients can videoconference with physicians on a host of maladies, from skin rashes and flu diagnoses to getting a prescription for an eye infection. On Maven, a telemedicine app targeted to women, nurse practitioners chat with patients on issues such as birth control, breast feeding and postpartum depression. Opternative offers online eye exams that it says are just as accurate as in-person tests. It’s a boom time for telehealth. Just as smartphone apps have revolutionized ride-hailing and apartment-sharing, telemedicine technology is upending health care.