How was alexander able to turn porus's elephants into a kind of weapon?

Alex's soldiers and cavalry were able to box in/encircle the elephants; many of the drivers  were killed by spears; elephants grew tired and many were wounded; w/o drivers they were  disorganized and turned every which way and trampled many Indian soldiers

Explanation:

disease was the number one killer of native peoples. they had had little exposure to the diseases of europe. this led to some villages being almost completely wiped out and some tribes destroyed.