Here is an interesting article about the role of disease in collapsing Rome. Unfortunately, there’s a bit on climate change, but we need not get too scared, since we can admit that in the past climate change did occur, having both good and bad effects, with the Vikings in Greenland probably getting wiped out because of a colder, not warmer climate.
“At some time or another, every historian of Rome has been asked to say where we are, today, on Rome’s cycle of decline. Historians might squirm at such attempts to use the past but, even if history does not repeat itself, nor come packaged into moral lessons, it can deepen our sense of what it means to be human and how fragile our societies are. In the middle of the second century, the Romans controlled a huge, geographically diverse part of the globe, from northern Britain to the edges of the Sahara, from the Atlantic to Mesopotamia. The generally prosperous population peaked at 75 million. Eventually, all free inhabitants of the empire came to enjoy the rights of Roman citizenship. Little wonder that the 18th-century English historian Edward Gibbon judged this age the ‘most happy’ in the history of our species – yet today we are more likely to see the advance of Roman civilisation as unwittingly planting the seeds of its own demise. Five centuries later, the Roman empire was a small Byzantine rump-state controlled from Constantinople, its near-eastern provinces lost to Islamic invasions, its western lands covered by a patchwork of Germanic kingdoms. Trade receded, cities shrank, and technological advance halted. Despite the cultural vitality and spiritual legacy of these centuries, this period was marked by a declining population, political fragmentation, and lower levels of material complexity. When the historian Ian Morris at Stanford University created a universal social-development index, the fall of Rome emerged as the greatest setback in the history of human civilisation. … The highly urbanised, highly interconnected Roman empire was a boon to its microbial inhabitants. Humble gastro-enteric diseases such as Shigellosis and paratyphoid fevers spread via contamination of food and water, and flourished in densely packed cities. Where swamps were drained and highways laid, the potential of malaria was unlocked in its worst form – Plasmodium falciparum – a deadly mosquito-borne protozoon. The Romans also connected societies by land and by sea as never before, with the unintended consequence that germs moved as never before, too. Slow killers such as tuberculosis and leprosy enjoyed a heyday in the web of interconnected cities fostered by Roman development.