In the Qinghai-Tibet plateau, one of the region’s highest risk areas for human plague, Himalayan marmots are the primary carriers of the infectious bacterium Y. pestis. Y. pestis infection can be transmitted to humans and other animals by the marmots’ parasitic fleas. Researchers determine that Tibetan sheep, who make up about one-third of China’s total sheep population, also carry this disease and can transmit it to humans.