[SlowBoat407]

(Some historians say that plague had swept through the area prior to the Arab advance and thus contributed to the sense of exhaustion & low morale.)

Was that not the plague spread by the mohemmedans launching the heads of murdered victims into the city?

[Reily]

The plague those historians were talking about was
pre-Arab conquest. It was probably was enabled by a breakdown in civil order due to:

1) The seemingly endless war between the Sassanid (Persian but not Muslim, Zoroastrianism) Empire & Roman(later the Byzantines),

2) The disorder caused by the Muslim conquest of the Sassanids. Another interesting point to remember when you here about the "Islamic Cultural Glory" is the fact that much of what later became known as Islamic culture, architecture, writing and other skills, were taken mainly from the Sassanid Persians into the broader Muslim world. Also much of the Mediterranean world they seized already had a cultural & technological foundation based on the Greek & Roman principles.

As far as heads being shot over the walls well maybe but I have never read anything connected to this "plague" & this terror tactic. The Persians are a fascinating people it would be wonderful if some day they could recover their ancient legacy.