The Ebola virus that has claimed many human lives in central Africa is also threatening the region's great apes, conservationists say.
By Alex Kirby
BBC News Online environment correspondent
More than 80 people have died this year in the outbreak, in the Gabon/Congo-Brazzavile border area.
There are now fears for one of the largest concentrations of western lowland gorillas.
Some scientists believe the virus may have killed thousands of apes in the last few years.
The warning comes from IUCN-The World Conservation Union, which represents 10,000 government and non-government scientists from 180 countries.
Dr William Karesh, of the US Wildlife Conservation Society (WCS), co-chairs the veterinary specialist group of IUCN's Species Survival Commission.
He believes the Ebola outbreak has affected tens of thousands of square kilometres over the last five or six years.
In that time it has killed hundreds of people, and Dr Karesh says there is "a real possibility" that thousands of great apes have also succumbed.
He said: "For years, many of us have been trying to point out that disease and health (whether wildlife, domestic animals, or human) are critical factors that have to be included in effective conservation planning."
Dr Jean-Christophe Vie, of IUCN's Species Programme, said: "Diseases affecting wildlife have not always been properly taken into account in conservation planning in the past.
Eight gorilla families vanished in two months
"Chimpanzees and gorillas are already endangered, and Ebola adds yet another threat to those already facing these species, such as deforestation and the wild meat trade."
Ebola haemorrhagic fever is described by the World Health Organization as "one of the most virulent viral diseases known, causing death in 50-90% of all clinically ill cases". The virus was confirmed in Congo in December 2002.
Six gorillas, all from one family group which had been followed by researchers for 10 years, were found dead at the time in a sanctuary covering roughly 11,000 square kilometres (4,250 square miles) in north-western Congo, near Gabon.
Local people have been involved in establishing the sanctuary as a protected area to prepare gorillas for the arrival of tourists.
At the end of January eight gorilla families were found to have disappeared over the previous two months: conservationists reported what IUCN calls "the quasi-disappearance" of the species from the sanctuary.
IUCN says primates are especially susceptible to many diseases affecting humans, apart from Ebola, because of their close relationship to us.
Gorillas face many threats besides disease (Image: Annelisa Kilbourn/WCS)
It says: "The transmission of the virus from the forest near the affected villages follows contact between hunters and the carcasses of great apes.
"Infected hunters have reported eating the dead gorillas and chimpanzees (although it is illegal to do so)."
Several organisations have been working for some years to monitor the health of the region's gorillas. They include WCS, Ecofac (Conservation and Rational Use of Central African Forest Ecosystems), and CIRMF (Primatology Centre, International Medical Research Institute, Gabon).
Dr Karesh said managing the problem was near-impossible because of the region's instability. He urged a programme of Ebola research and prevention.
Lowland gorillas, which are classed as endangered, live in tropical rain forests in the DRC, Congo-Brazzaville, Gabon, Equatorial Guinea, Cameroon, and the Central African Republic.