The die-off is now 10 times bigger than the 2010 event. And because the saiga population was at a precariously low level, the die-off has claimed an astronomical proportion of the species, from one-third to perhaps a half.

“The scale is absolutely unprecedented,” Dr. Kühl-Stenzel said.

As soon as the reports of the die-offs began emerging, Dr. Kock and other wildlife disease experts swung into action, traveling to Kazakhstan to study the outbreak as it unfolded. They examined dead animals, performing necropsies on 15 of them.

Dr. Kock was astonished by the deadliness of the disease, whatever it is: Once it struck a herd, every animal died, in a matter of days.

“It is an extraordinary thing to get 100 percent mortality,” Dr. Kock said.

He and his colleagues found that the saigas were infected with two species of deadly bacteria, Pasteurella and Clostridium. But Dr. Kock says he suspects that these infections became deadly only when something else crippled the animals.

There are two reasons for this suspicion. One is the speed with which the animals died: so fast that they would not have enough time to spread a virulent strain of Pasteurella or Clostridium to other animals.

And Pasteurella and Clostridium are common in healthy animals. Only when the animal becomes weakened do these microbes turn deadly.