DAKAR, Senegal — The worst Ebola outbreak in history took a big step toward ending on Tuesday when Guinea, the only nation where the virus had been lingering, began its official countdown to being declared free of the disease.

The countdown began after a 3-week-old girl, Nubia Soumah, the last known patient in active treatment in the world, tested negative for the virus twice in a row.

Dozens of people in Guinea are still being monitored to see if they develop symptoms of the virus. But if no further cases emerge in Guinea over the next 42 days, the equivalent of two incubation periods for the virus, the nation will be declared officially free of Ebola, nearly two years after the epidemic began in the country’s forest region.

Nubia contracted the virus from her mother, an Ebola victim who died after giving birth. For several days, the baby has been the sole patient at a Doctors Without Borders facility in Conakry, the capital. She will be closely monitored at the facility for follow-up care because of her fragile age.