The cause of death of nine chimpanzees who died mysteriously at the Tai National Park in Ivory Coast between 2001 and 2002 has been found to be anthrax - a development that has put the bacteria on the list of extinction threats to Africa's great apes.
Nine of the 79 Tai chimps were killed by the anthrax
The findings were confirmed by Dr Heinz Ellerbrok of the Robert Koch Institute in Berlin and Dr Fabian Leendertz of the Max Planck Institute, and published in the journal Nature.
Dr Leendertz told BBC World Service's Science In Action programme that the chimps had often displayed no signs of being unwell until only very shortly before their deaths.
"One... suddenly had troubles getting up, vomited two times, and climbed up a little tree and fell down dead.
"That was it - no real symptoms."
Anthrax now joins deforestation, the bushmeat trade and other diseases such as Ebola on the list of extinction threats to the apes.
Dr Leendertz said that Ebola was among the suspected causes of the ape deaths, as it is also very swift to kill - but that because the symptoms were so unclear it took a long time to identify the anthrax as the pathogen.
Meanwhile Dr Ellerbrok said that anthrax had never been before seen in wild primates or in the tropical rainforest environment.
Anthrax cannot pass from chimp to chimp
"We knew that anthrax can affect great apes, but it has never been observed in wild primates and it has never been observed in this particular setting," he added.
"This bacteria has never been discovered before in tropical forests. It was a big surprise for us. It took us quite a while to think about anthrax."
They said that the key question remained of where the anthrax had come from.
"We know it's there, but we have no idea where it came from - whether it has been residing in this region for ages, or whether it was introduced recently from neighbouring countries."
The mystery is further deepened because anthrax cannot be passed on from chimp to chimp.
"You have to ingest the germs in one way or the other - either inhaling spores, or consuming infected meat or skin lesions," Dr Ellebrok explained.
One possibility is the meat the chimps gather from hunting, as they eat a number of different monkey species.
However, the chimps had been under constant observation and none had been seen hunting in the days preceding their deaths.
Dr Ellerbrok also suggested that it was possible an animal that had been killed by anthrax had lain dead in a water source, and the chimps had picked up the anthrax from there.
"There must be a common source," he stressed.
"In two of the three outbreaks we observed, several animals were affected at the same time."
Ongoing conflict in Ivory Coast has also taken its toll on the chimps
Meanwhile, there are now concerns that, as there is an ongoing illegal trade in bushmeat throughout the region, the anthrax bacteria could be passed on to humans.
"This is one big concern," Dr Ellerbrok said.
"It is not only anthrax - we have found other pathogens and retroviruses in the forest.
"All these pathogens can be transmitted via bushmeat consumption and handling the meat."
He added that if anthrax did jump to humans, the Ivorian government would prepare a campaign to warn of the dangers of bushmeat consumption.
"We are working in close collaboration with Ivorian institutes," he said.
"We will be looking for pathogens... to see if this transmission has already taken place."