In Moshe's warning to Pharaoh (in Hashem's name) of the fourth plague, *[arov](https://judaism.stackexchange.com/questions/13458/what-was-the-arov)*, he adds ([Ex. 8:18-19](http://mechon-mamre.org/i/t/t0208.htm)):

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> וְהִפְלֵיתִי בַיּוֹם הַהוּא אֶת-אֶרֶץ גֹּשֶׁן אֲשֶׁר עַמִּי עֹמֵד עָלֶיהָ לְבִלְתִּי הֱיוֹת-שָׁם עָרֹב... וְשַׂמְתִּי פְדֻת בֵּין עַמִּי וּבֵין עַמֶּךָ
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> "I will distinguish at that time the land of Goshen, where My people are located, that there shall not be *arov* there... I will make a separation between My people and yours."
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Similar statements are made concerning the plagues of pestilence ([9:4](http://mechon-mamre.org/i/t/t0209.htm)) and the death of the firstborn ([11:7](http://mechon-mamre.org/i/t/t0211.htm)). About the hail and the darkness, Moshe doesn't say anything to this effect, but the narrative records that the Jews weren't affected (9:26, [10:23](http://mechon-mamre.org/i/t/t0210.htm)). Nothing is said explicitly regarding any of the other plagues.

Why is this? According to the Midrash ([Shemos Rabbah 9:10](http://he.wikisource.org/wiki/%D7%A9%D7%9E%D7%95%D7%AA_%D7%A8%D7%91%D7%94_%D7%98_%D7%99)) that, for example, the plague of blood didn't affect the Jews either, why is this "distinction" spelled out only beginning with *arov*? Conversely, are there any sources that suggest that some of the plagues indeed did affect the Jews, and that they were exempted only from the ones where it's specified otherwise?