One day in 2018, Apple vice president Sumbul Desai received a letter from a woman thanking Desai for saving her life. The woman recounted how, a few weeks after having a baby, she had received an alert from her Apple Watch indicating that her heart rate was abnormally high. This prompted her to go to hospital, where she was diagnosed with thyroid storm, a condition that elevates heart rate and body temperature to dangerous levels and, if left untreated, can be fatal. “Had she not paid attention to it, her outcome could have been much worse,” Desai recalls. “Her letter is incredible moving because she talks about how she’s with her child [and] she’s healthy now.”

Desai, who joined Apple’s health team in 2017 after a stint as head of Stanford University’s Centre for Digital Health, regularly receives similar letters, and many such stories have surfaced online. There was the case of Scott Killian, for instance, who, after being given a clean bill of health a few months before turning 50, was woken up one day at 1AM by his Apple Watch alerting him that his heart rate was very high. During his subsequent visit to the emergency services, Killian was told by a doctor that he had suffered a heart attack and had four blocked arteries. He would probably have died if he hadn’t had the alert.