Spirited, slightly weird-looking and instantly recognizable, the Saiga antelope find safety in numbers during their spectacular mass migrations. But since the early 2000s, they’ve been considered critically endangered. Now, the fragile antelopes are doing something else en masse: dying. As the BBC’s Victoria Gill reports, a quarter of the world’s saiga population is thought to have died in Mongolia.

It’s devastating news for a species whose existence is already under threat. Scientists tell Gill that ovine rinderpest, a disease also known as sheep plague, Peste des Petits Ruminants or PPR, is to blame. According to the Food and Agriculture Organization of the United Nations, the highly contagious disease can affect up to 90 percent of an animal herd and kills up to 70 percent of the animals who contract it. PPR is viral and has a range of symptoms, including fevers, stomach problems and pneumonia among others. It is spread by close contact between animals—and for free-ranging creatures like antelope, who are not managed by farmers or keepers, it can rage unchecked.

The news is especially devastating for Saiga antelope, whose numbers are already so low the entire species is considered critically endangered by the IUCN. Though a population of at least one million is thought to have existed as late as 1994, their numbers have since dwindled. The animals were poached into oblivion by hunters who sought their horns to sell them to Asian countries for medical use. As The New York Times’ Erica Goode reports, only 50,000 Mongolian saiga are thought to live today.

This isn’t the first time saiga have been wiped out. In 2015, nearly half of the global population—over 120,00 animals—died over the course of just two weeks. Though the cause was initially a mystery, scientists and conservationists now think it was due to a bacterial infection. Altogether, 95 percent of the animals have been lost in just a decade.

How can the potentially disastrous epidemic be halted? As Gill reports, animal carcasses are being burned to prevent PPR from spreading. But the animals that do survive could be weak and susceptible to other diseases and conservationists worry that the species could now be doomed. That’s horrible news not just for the antelope, but for the ecosystem of the grasslands where they live. Other animals could catch PPR, and endangered snow leopards, who rely on saiga for food, could suffer, too. The race is on to eradicate PPR and save these strange-looking antelopes from extinction.