McEvedy C
Sci Am. 1988 Feb;258(2):118-23. doi: 10.1038/scientificamerican0288-118.
In A.D. 1346 some 100 million people inhabited Europe, northern Africa and the Near East. Five years later 25 million were dead--victims of the Black Death. The plague kept reappearing, but the epidemics did not spread as widely: apparently a new and milder strain of Yersinia pestis evolved that made at least some people immune to the virulent strain.
公元1346年，欧洲、北非和近东地区约有1亿人口。五年后，2500万人死亡，成为黑死病的受害者。鼠疫不断复发，但疫情没有蔓延得那么广泛：显然，一种新的、毒性较弱的鼠疫耶尔森菌菌株进化出来，使至少一些人对毒性较强的菌株具有免疫力。
