The $12 billion health care institution known as Mayo Clinic has a tradition of rotating out top leadership every seven to 10 years. Current CEO John Noseworthy was asked to break the trend and stay on, but with nine years at the top, he picked the last day of 2018 as his time to exit. Taking the reins will be Gianrico Farrugia, a 30-year veteran of the Rochester nonprofit, who has run its Florida operations for four years. Like his predecessor, Farrugia came in as a medical professional (he’s a gastroenterologist). He’ll be tasked with fine-tuning Mayo’s structure, including the organization’s $800 million expansion plan for Arizona and Florida and its $6 billion Destination Medical Center build-out in its home city.