William H. McNeill
"William H. McNeill (b.1917), a world historian, noted in chapter three of his book Plagues and Peoples (1976) that the Roman Empire suffered the severe and protacted Antonine Plague starting around 165 A.D. For about twenty years, waves of one or more diseases, possibly the first epidemics of smallpox and/or measles, swept through the Empire, ultimately killing about half the population. Similar epidemics also occurred in the third century. McNeill argues that the severe fall in population left the state apparatus and army too large for the population to support, leading to further economic and social decline that eventually killed the Western Empire. The Eastern half survived due to its larger population, which even after the plagues was sufficient for an effective state apparatus."
"This theory can also be extended to the time after the fall of the Western Empire and to other parts of the world. Similar epidemics caused by new diseases may have weakened the Chinese Han empire and contributed to its collapse. This was followed by the long and chaotic period sometimes called the Chinese Dark Ages. Later, the Plague of Justinian may have been the first instance of bubonic plague. It, and subsequent recurrences, may have been so devastating that they helped the Arab conquest of most of the Eastern Empire and the whole of the Sassanid Empire. Archaeological evidence is showing that Europe continued to have had a steady downward trend in population starting as early as the 2nd century and continuing through the 7th centuries. The European recovery may have started only when the population, through natural selection, had gained some resistance to the new diseases..."