Photo Credit: https://en.wikipedia.org/wiki/Saiga_antelope
This virulent mysterious epizootic illness caused herd fatality of 100% once they were infected. It is estimated that more than 120,000 saiga were lost, some 40% of the species' total worldwide population of 250,000.
The deaths have been attributed to the Pasteurella bacterium an opportunistic pathogen that weakens the immune system. It seems also likely that a number of environmental stress factors were involved that made the Saiga susceptible to the bacterium. An international team has been put together to investigate the deaths.
Source: For more information on the species see Saiga Conservation Alliance publication