There is a recent trend in fiction works such as I Am Legend, The Last of Us, the Planet of the Apes reboot, where an epidemic that kills 70-90% of the world's population is able to brought it to post-apocalyptic wasteland level with no need of nuclear war like in past fiction (let's ignore straight zombie fiction and alien invasions to keep the death-from-germs thing). But has something remotely like this actually happened to a country or civilization in History?
i don't know about epidemics, eventually people start to be resistant to particular virus strains or whatever else has caused the outbreak, however climate change can change things significantly:
A century long drought is thought to be a major reason for the collapse of the Akkadian empire and of the Egyptian old kingdom.
http://en.wikipedia.org/wiki/4.2_kiloyear_event
http://en.wikipedia.org/wiki/Akkadian_Empire#Collapse_of_the_Akkadian_Empire