Iraq invaded Iran on 22 September 1980, triggering a bitter eight-year war which destabilised the region and devastated both countries. The then Iraqi leader Saddam Hussein claimed as a reason for the invasion a territorial dispute over the Shatt al-Arab, the waterway which forms the boundary between the two countries.
I believe the answer is smallpox, but I could be mistaken. There were lots of viruses threatening the population at the time but from what I know, smallpox was the largest threat.
