When Patty Stonesifer tried to stop working for Bill Gates a decade ago, he had to meet some unusual conditions to keep her. She wanted no pay and no involvement in business but a big expansion in his fledgling non-commercial activities. Neither she nor he could imagine how big the result would be.

Now she is leaving again, this time from her post as chief executive of what has become the Bill and Melinda Gates Foundation. As Stonesifer heads to the Smithsonian Institution, she bequeaths Gates the world’s largest philanthropic organisation, the legacy of which – both its strengths and weaknesses – is almost as much hers as his.