Yunnan Sudden Death is an unexplained sudden death syndrome that occurs primarily in northwest Yunnan Province, P. R. China. Yunnan Sudden Death Syndrome is a serious threat to the minority people living in the region. According to incomplete statistics, from 1975 to 2010 there were 386 cases of sudden death in more than 80 villages in 24 counties in Yunnan province. It differs from other sudden death syndromes by three characteristics: (1) Special clustering (family clustering or village clustering). Within a restricted area numerous occurrences of sudden death have been reported. In certain villages multiple sudden deaths have occurred at the same time or during an interval of just a few days. Many of the deaths occur at night; (2) Yunnan Sudden Death occurs mainly after the summer rainy season in July and August; (3) there are no common pathological manifestations of Yunnan Sudden Death Syndrome. The most common cause of death is cardiac arrest in adults over the age of 30. However, detailed epidemiological, clinical, anatomy, pathology, and virology studies failed to identify the cause of the sudden deaths. Moreover, in some instances there was sudden death of livestock at the same time as the human sudden deaths.

One common factor is that all the afflicted villages are sited on a major geologic fault. Exceptionally high levels of carbon dioxide have been measured at the fault in a nearby region. It is possible that the rain water flushes the carbon dioxide from the fault zone flooding low-lying areas and causing the sudden deaths. This situation would be especially dangerous for the many people who sleep on floor mattresses.