Why does the Torah use the expression, "'Al Devar," "on the word, of Sarai (Sarah)"?
Targum Onkelus, Targum Yonasan and Ramban: The phrase simply means "due to the matter of," i.e. because of what Pharaoh did to Sarah (and Avraham - Ramban).
Rashi: As is usually the case, the words, "on the word of" mean literally that Sarah instructed the angel (that was protecting her) to strike Pharaoh, and he struck. 1
Gur Aryeh: Had the verse intended the first explanation, it should have written, "Al Odos Sarai."
Seeing as it was only Pharaoh who sinned, why did HaSh-m punish the members of his household?
Seforno: In fact, it was only Pharaoh himself who was stricken with "great plagues"; the members of his household suffered minor ones. This was so that the only person in the palace who did not suffer was Sarah, and they would know without doubt, that it was all due to Sarah - and they would be encouraged to do Teshuvah.
How did Pharaoh deduce that Sarah was a married woman?
Gur Aryeh: The verse implies that only Pharaoh himself received Ra'asan, while the rest of his household received other plagues. He realized that he was singled out in order to prevent him from being intimate with Sarah.
Why did HaSh-m strike Pharaoh? He was Shogeg (unaware [that she was married])! Additionally, we find that HaSh-m warned Avimelech before striking him (20: 3,7), yet He did not warn Pharaoh?
Moshav Zekenim: Initially Pharaoh was Shogeg, but later he was Meizid after HaSh-m told him that she was Avraham's wife. She herself would not tell, for Avraham commanded her not to.
Riva: Avimelech was also stricken; all of his openings were closed. There is no source to say that Pharaoh was stricken more.
QUESTIONS ON RASHI
Rashi writes: "Great afflictions (Nega'im) - Pharaoh was afflicted with Ra'asan, which made relations difficult." How do we know it refers to Ra'asan?
Gur Aryeh #1: Why was he afflicted with Nega'im, and not other Chola'im (illnesses)? It must have been to make relations difficult for him.
Gur Aryeh #2: The verse implies that only Pharaoh himself received the "great afflictions," while the rest of his household received other plagues. He must have been singled out in order to prevent him from being intimate with Sarah.