NAIROBI — About a year and a half after it emerged, the Ebola outbreak in Sierra Leone has officially ended.

More than 11,000 Sierra ­Leoneans died of the disease, and at least 17,000 survived it. It came upon some slums and villages like a wave, infecting scores overnight and posing one of the most significant public health challenges in recent history.

Global health officials and members of the country’s government gathered Saturday in the capital to mark the milestone. The World Health Organization said 42 days had passed since the last recorded case, the agency’s criteria for declaring the end of an outbreak.