The World Health Organization has asked Russian authorities for more information on the cause of death of a woman who worked at a plague research institute in Siberia and died after contracting severe pneumonia.
Disease experts say that incomplete and sometimes contradictory information from Russia's health authorities makes it impossible to draw firm conclusions about the risk to the wider population, as fears rise over the potential spread of an unknown illness.
Here's what we know so far.
How much does this threaten the world?
Russian authorities told the WHO that no case of plague had recently been registered in the Irkutsk region where the woman died and that no "high-threat pathogens" were found among her contacts.
They have said little else about the incident.
The WHO has asked Russia for more information on the pathogen that prompted the public health measures put in place in the Siberian region, including close monitoring of the woman's contacts.
The United Nations agency has also asked for clarification about media reports that a second employee had contracted pneumonia of undetermined cause.
If the Russian woman died of pneumonic plague, as some initial reports suggested, others she had contact with could have been exposed. But scientists say it is unlikely to have spread widely around Russia or to other countries. That is mostly because the bacteria that cause the disease require close contact to spread, symptoms develop quickly and the disease can be treated with antibiotics.
Given the available information, scientists say it is encouraging that no other cases have been reported, though that doesn't mean they haven't happened.
What have Russian officials said about how the woman died?
The Russian government said the woman died of "pneumonia of an unknown cause." In other words, they are saying they don't know what pathogen caused the infection of her lungs, let alone whether it was Yersinia pestis, the bacteria that causes the plague.
How long was she ill?
The 28-year-old, who worked at the Antiplague Scientific Research Institute in Irkutsk and was named in local media as Daria Shipilova, died on Oct. 2, a few days after falling ill, according to local and independent media.
What is plague and how do you catch it?
The disease, known as the "Black Death" in the Middle Ages, once killed millions of people in Europe. Today, plague is rare but the bacterium that causes it still exists in parts of Africa and Asia, including Siberia.
Humans usually contract it from fleas that have fed on infected rodents, or sometimes by handling the tissue or body fluids of a diseased animal, according to the U.S. Centers for Disease Control and Prevention. About seven cases are reported a year in the U.S., mostly in northern New Mexico and Arizona. Globally, a few hundred cases are reported each year, experts say.
How deadly is the plague and can it be treated?
Today, the plague can be cured with antibiotics, but only if treatment is given quickly. The disease is almost always fatal if treatment doesn't begin within 24 hours of the first symptom, particularly if it reaches the lungs as pneumonic plague.
What about pneumonic plague?
This is the most dangerous and contagious form of plague, infecting the lungs, and the only one that can be spread person to person. Respiratory droplets carrying the Yersinia bacteria are released through coughing or sneezing.
Person-to-person spread of pneumonic plague hasn't been documented in the U.S. since 1924, according to the CDC, but there have been rare cases among people exposed to cats that were sickened by eating infected rodents.
The other forms of plague are bubonic, which infects the lymph nodes, and septicemic, which is transmitted in the bloodstream and is also very deadly. Those forms of the disease can develop into pneumonic plague if the bacteria spread to the lungs.
Does pneumonic plague spread as quickly as Covid-19?
Transmission requires close, face-to-face contact -- usually within a 6-foot radius. Respiratory droplets containing Yersinia pestis are heavy and drop to surfaces quickly. So the bacteria doesn't spread anywhere near as easily as the flu or Covid-19 virus.
But the disease can spread quickly because the incubation period is very short -- symptoms can appear within a day and up to a week later -- and an infected person's cough can be teeming with bacteria.
What are the symptoms of pneumonic plague?
Patients quickly develop a fever and headache and feel weak, and then rapidly progress to shortness of breath, chest pain, cough and sometimes bloody or watery mucus, according to the CDC.
What about people who have been exposed to pneumonic plague but don't show symptoms?
People who have had close, face-to-face contact with a patient are given preventive oral antibiotics to stop an infection before symptoms appear.
What measures are Russia's neighbors taking?
Kazakhstan, which shares the longest land border with Russia, has imposed temperature checks for those crossing the border, the country's state-controlled news agency said. Kyrgyzstan and Uzbekistan, neither of which shares a land border, maintain heavy air traffic with Russia and have stepped up passenger monitoring, their health ministries said.
What are U.S. authorities doing?
The U.S. has warned Moscow that its failure to disclose details about the death runs counter to its international health obligations, the Journal has reported.
President Trump told reporters at the White House that he has a call scheduled with Russian President Vladimir Putin soon. When asked what U.S. intelligence agencies are telling him about the incident, the president said he should be able to share more Wednesday.
What's Russia's track record in this area?
Trust in Russian authorities is low following previous health crises, including the Covid-19 pandemic, in which the government was accused of playing down the number of deaths. Soviet authorities famously denied a leak of anthrax spores in 1979, blaming tainted meat. Russian President Boris Yeltsin admitted in 1992 that the disaster was caused by an accident at a military germ-warfare facility in the city of Sverdlovsk, now known as Yekaterinburg.
This explanatory article may be periodically updated.