In history there have been many accounts of plague, either from disease, famine, or war. When was the first ever plague? I'm writing a paper on the history of medicines and I'm trying to figure out the earliest forms of plague.
It looks like the current champion for first recorded plague or pandemic was an outbreak that hit the Middle East in the time of Pharo Akhenaten (roughly 1600 BCE). There is a lot of argument over what exact disease it was, with bubonic plague, influenza, and polio all being argued for.
Ankh Nfr has a long discourse on the evidence for this plague and what its source may have been at AmarnaLover. If you want details, it may be worth a read.
Note that we believe the most common temperate infectious diseases evolved for human tranmission amongst the densely populated agricultural areas. So it would make sense that such diseases might start appearing alongside the first agricultural socieites. Likely they were occurring far before anyone had invented writing.