The Ebola outbreak in Congo passed a grim milestone this week — 10,000 confirmed cases — and spurred questions about whether health authorities are losing their ability to contain it.

Nearly half the infected people in this outbreak have died, and some experts say there are reasons to worry this could grow into the worst Ebola outbreak in history. Others don't think it will reach that level. Researchers are currently working on updated projections.

A Swiss woman being treated for Ebola in Nigeria was medically evacuated to Switzerland with her husband Monday, officials said, as Nigerian authorities continued monitoring 52 people who had been in contact with her.

Nigerian Health Minister Muhammad Pate said the woman, the country's first case in the current outbreak, left the capital, Abuja, early Monday with his approval. Her condition had improved significantly, and she was stable, conscious and able to communicate, he said.

Ebola disease from the fast-moving outbreak in Congo has now spread to four other countries, including two where it's active. It has spread farther across Africa than any previous Ebola outbreak, reaching three regions.

Nigeria on Tuesday became the latest country affected by the outbreak, which was first declared in eastern Congo in mid-May. Caused by the rare Bundibugyo virus for which there's no approved vaccine or drug, the outbreak has killed at least 4,600 of the 9,500 people infected in Congo.

Crammed with beds and stretchers, the dilapidated Al Jumhuri hospital in Yemen's besieged city of Taiz is running so low on fuel and medicine that it may have to close, officials warn.

The Kremlin has long maintained a culture of secrecy when it comes to controlling public information, especially involving accidents -- from industrial disasters to health emergencies.

The suspicious death last week of a worker at a research lab in Siberia that has fueled global concerns about suspected pneumonic plague is just the latest example of how leaders in Moscow manage the official narrative that often covers up the truth for years.

Russia's public health watchdog on Thursday denied that a lab worker who died in Siberia had spread infection to another person, as global concerns swirled about suspected pneumonic plague.

Rospotrebnadzor has said that the lab worker at a plague research institute in Irkutsk in southeastern Siberia died of pneumonia of unknown origin. In tersely worded statements, it said that "extensive" testing found no evidence her illness was caused by any of the pathogens she worked with, and it informed the World Health Organization that no case of plague has been registered in the Irkutsk region.

The head of the World Health Organization said Wednesday that Russian authorities have reported no case of plague in a Siberian region where a woman employee of a laboratory that studies the disease died of pneumonia last week.

The comments from Tedros Adhanom Ghebreyesus come a day after WHO said a possible case of pneumonic plague in Russia presents a "very low" risk outside the country as investigators looked into what caused her death Friday.

The World Health Organization said Tuesday that it deemed the public health risk across Russia to be low, following reports that a laboratory technician had died of plague.

Moscow on Monday downplayed reports that a laboratory technician had died of plague, stressing that post-mortem tests had not detected the disease, while Washington said it was closely monitoring the situation.

The Nobel medicine prize was awarded Monday to U.S. psychiatrist and neurologist Karl Deisseroth and German scientists Peter Hegemann and Georg Nagel for their pioneering work in the neuroscience field of optogenetics.