What deeds that are 'not normally done' did Avimelech and his subjects suffer?
Rashi: HaSh-m blocked all their orifices - those of the Zera, of Ketanim and Gedolim, the ears and the nose.
What was Avimelech accusing Avraham of?
Seforno: He was accusing him of causing so much damage to a person whom he did not know and with whom he had not fallen out. Moreover, he gained nothing from the harm he was causing him.
QUESTIONS ON RASHI
Rashi writes: "'Deeds that are not to be done' - An unheard-of plague has come upon us." Why not explain simply, that Avimelech accused Avraham of having acted improperly?
Gur Aryeh: Avraham had not committed any "deeds" against them, but rather only one deed -namely, he said that Sarah was his sister. Nor was this even a deed, but merely a statement.
