Russian authorities investigating the death of a woman who worked at a plague research institute in Siberia are coming under growing international pressure to explain how she died.
"It's incumbent upon Russia to obviously share more information with the world," Secretary of State Marco Rubio said Tuesday during a visit to Iceland. "That's what they should do."
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 World Health Organization on Tuesday -- four days after the woman died -- that it wasn't aware of any of her contacts experiencing "symptoms of illness."
They have said little else about the incident.
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 by antibiotics.
As time passes since her death and no related fatalities or illnesses are reported, the likelihood grows that either the woman didn't have the plague or didn't transmit it to others. The institute where she worked studies other diseases. She also could have been exposed to an illness outside of work.
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.
"Plague comes on very quickly," said Samuel Scarpino, a professor of public health and health sciences at Northeastern University. "If it was plague, we would have seen more cases by now if there were transmission."
What have Russian officials said about how the woman died?
The Russian government has 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 cause the plague.
Contacts of the woman are under close medical observation. On Tuesday, Russian authorities said that among them they have identified two cases of Covid-19 and two of rhinovirus, an infection that leads to the common cold.
Fears were initially raised when some regional officials and media reported that the woman died of pneumonic plague, which can spread person to person. Lack of information from the Russian government has further stoked anxiety about the nature of her death.
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 is caused by Yersinia pestis bacteria. Known as the "Black Death" in the Middle Ages, the plague once killed millions of people in Europe. Today, cases are rare but the plague still exists in parts of Africa and Asia, including Siberia.
Humans usually contract it from fleas that have fed on infected wild 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. When large numbers of rodents become infected and die, the fleas seek other sources of food -- including humans. 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 form that can be spread from one person to another. 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?
President Trump said that the U.S. was looking at the situation in Russia "very strongly" and that the U.S. was willing to help, though he said he hadn't been in contact with Russian President Vladimir Putin on the matter.
What's Russia's track record in this area?
Trust in Russian authorities is low following previous health crises, including the coronavirus 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 Yekaterinburg.)
This explanatory article may be periodically updated.