If not, is there a known reason why? If so, what impact did the resulting demographic decline have on China's political stability?
There is considerable disagreement on the possibility of pre-modern plague epidemics in China.
This excellent writeup by u/y_sengaku talks about the possibility of Yersinia Pestis (often called plague or bubonic plague) reaching China, arguing that it was possible
This post helpfully links to several excellent posts by u/mikedash answering similar questions, arguing that it is unlikely. I have a few quibbles with minor points in Mike Dash’s post which I discuss here
I come down firmly in the first camp, I believe that Yersinia Pestis epidemics or pandemics did occur in China prior to the well-recorded modern pandemics. But I will admit that the evidentiary record is limited.
The study of Yersinia Pestis (YP hereafter) has been transformed at a dizzying speed in the past decade by paleogenetic evidence from archeological samples. In which plague DNA can be identified from the tooth pulp in exhumed skeletons. This can be used as evidence that people died of YP infections, but the genetic code can also be subjected to analysis to trace the evolution of the pathogen and identify when genetic branches split off; data which can used to identify where and when epidemics occurred.
This data has upended many traditional ideas about YP, the study of which previously was very reliant on textual evidence. The Paleogenetic evidence indicates that YP was already present at both ends of the Eurasian landmass more than 5,000 years ago (found in ancient skeletons in both Latvia and the Tian Shan mountains). And indicates that Eurasia functioned as a unitary disease system, with strains continuing to travel back and forth across the continent as recently as the sixth century AD, with the same strain responsible for the Justinianic YP pandemic in Europe being found in Xinjiang.
Y. Pestis survives between human outbreaks in wild rodent populations, and is readily transmissible between rodents, humans, and other disease vectors such as fleas. More than 200 mammal species (mostly rodents) can be infected with YP. “Plague reservoirs” of infection among wild rodent populations exist at modern in the Western United States, Central Asia, southern Africa, India, and a variety of other locations. Cases of plague arise when humans come into contact with, eat, or are bitten by infected rodents.
Around the 13th century, YP underwent a wave of genetic mutation which gave rise to four distinct genetic lineages (see Green, “The Four Black Deaths” (2020)). Green’s narrative, which I find very convincing, is plague being spread across Eurasia by Mongol movement in the 13th century, eventually sparking the 14th century plague outbreak in Europe (the Black Death). This movement also aligns roughly with a period of population collapse in China in which plague could be a contributing or primary factor. The scale of the mutation into four distinct lineages is indicative of a wide and sudden diffusion of YP, on the level of a pandemic.
Largely on the basis of this paleogenetic evidence, I believe there were YP epidemics in China which killed 30-60% of the population (like in Europe) but these epidemics do not occur in the written sources to any great extent. As y_Sengaku helpfully notes, the primary sources typically only refer to a "disease (疫)" and do not specify a symptom profile or particular disease. In some cases these epidemics are noted for having unprecedented mortality, with "nearly half" of the population dying. Mongol sieges in China, the middle east, and modern Iran were also accompanied by a pestilence with unprecented mortality.
But there is no 'smoking gun' literary evidence of buboes, the lymphatic swellings which are the classic identifier of YP. This is in contrast to both the sixth century and 14th century plague pandemics in Europe, where the unique symptoms of the disease are often remarked upon by the written sources. The prevalence of cremation in Chinese Burial practices also makes it unlikely that that we will get tooth pulp traces of YP of the sort which confirmed the Justinianic plague was YP.
Green's hypothesis is also complicated by the notion of the pandemic being spread out of a central asian disease reservoir by Mongol conquest. The population collapses and contractions in many areas can just as easily be ascribed to violence of the Mongol expansion. Though alternatively this violence could be concealing massive disease mortality from YP.
Chinese demographics for this period are a whole questions of their own (check this range of possible estimates), but in general it is believed the population was roughly halved between 1200 and 1400 AD. There is some indication this population collapse could have been concentrated in the twelfth century, so a Chinese YP pandemic could have been decades or even a century removed from the European Black Death. But it is hard to tease out the effect of disease from the extremely messy end of the Song period.
So While I do think that there was a Chinese YP pandemic, I will readily admit there are strong arguments for the opposite case.
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