Renee Dua and Nick Desai, the married founders of a fast-growing startup, were exhausted after their third child arrived late last year. But when it came time to set up and get her to her first pediatrician appointment, Desai whipped out his phone and opened an app. A few taps and swipes brought a doctor to their door the next day.

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Desai was using Heal, created by the startup he and Dua founded in 2014, in Los Angeles. The app allows patients and caregivers to hail a doctor seven days a week, from 8 a.m. to 8 p.m., for pretty much any service a primary-care physician can provide. Visits are covered by insurance or a flat $99 fee. Medical assistants, who are Heal employees or contractors, drive the docs to their 12 to 14 appointments a day in company-provided cars. In the homes of chronic disease patients, the company installs a "Heal Hub"--a device that checks vital signs and has a two-way radio that connects patients directly to a doctor.

Heal, which expects $35 million in revenue this year, operates in California's biggest cities and in Washington, D.C. New York City will be next, and Dua and Desai aim to cover the nation's 25 biggest cities by the end of 2020. "We can move into a new city and be up and running in 120 days," says Dua. "We've McDonald's-ized it." 

Correction: an earlier version of this story misstated the price of a Heal visit that's not covered by insurance. Those visits are $99.