First biological weapons: 3300 years ago

Ancient Middle Eastern texts suggest that more than 3,300 years ago the Hittites may have sent rams infected with a brutal bacterial infection to their enemies as a form of biological warfare. According to researcher Siro Trevisanato, the disease Tularemia, also known as rabbit fever, jumps between species via insects. Form New Scientist:

He believes tularemia is to blame for a deadly epidemic dubbed the "Hittite plague" which raged through the Middle East in the 14th century BC. Around 1335 BC, letters to the Egyptian king Akhenaten reported a pestilence in Simyra, a Phoenician city near what is now the border between Lebanon and Syria.

The (letters from 1335 BCE to the Egyptian king Akhenaten) describe a terrible illness causing disabilities and death. Most tellingly, they mention that, because of the plague, donkeys were banned from being used in caravans.

According to Trevisanato, this indicates that the people living in the city were hit by tularemia. The disease can infect donkeys and the insects that they carry, so preventing the use of donkeys for transport may have been an attempt to quell its spread.