The plague in Rome killed thousands of people, including soldiers, and laborers. This led to a corruption of barbarians and a decline in trade and agricultural production.
As invasions and wars increased in Rome, the amount of land available decreased. This led to over worked soil, and a major decline in the economy from the lack of agricultural output.
When Diocletian became the emperor in 284 he doubled the size of the army. Having to pay for all the soldiers impacted the economy very negatively, contributing to the fall of the empire.