Alexandria was joining the Hellenistic world to India and China as Buddhism was taking route. So, buddhism took route in Afghanistan while it was still part of the Hellenistic empires of Parthia and Seleucid. A Parthian Noble from Central Asia was responsible for the first Buddhist translations into Han Chinese the 100s B.C. I wonder what happened to the Buddhist missionaries that were sent to the Semites in Mesopatamia where the Parthians also ruled. Unfortunately we have no records. We do know that around this time, a philosophically complex, monotheism called Zurvianism had emerged within Zoroastrian thought. It the last paragraph we see the first evidence of the Viral Missionary structure that the Manichaeans used in the 200s A.D. The Manichaen Missionary Movement was apparently based on the Buddhist fourfold partitioning of the Community (Sangha). 

Overland Trade and Cultural Exchange from Antiquity to the Fifteenth Century by Richard Foltz http://books.google.com/books?id=czVRPa7GxrUC&lpg=PA37