"Lo Meis Ad Echad."

.וַיָּמָת כֹּל מִקְנֵה מִצְרָיִם וּמִמִּקְנֵה בְנֵי יִשְׂרָאֵל לֹא מֵת אֶחָד.I

and all the livestock of the Egyptians died, but of the livestock of the children of Israel not one died.

ז. וַיִּשְׁלַח פַּרְעֹה וְהִנֵּה לֹא מֵת מִמִּקְנֵה יִשְׂרָאֵל עַד אֶחָד

7. And Pharaoh sent, and behold, not until one of the livestock of Israel died

The term, "ad echad," is a little ambiguous.

 כח. וַיָּשֻׁבוּ הַמַּיִם וַיְכַסּוּ אֶת הָרֶכֶב וְאֶת הַפָּרָשִׁים לְכֹל חֵיל פַּרְעֹה הַבָּאִים אַחֲרֵיהֶם בַּיָּם לֹא נִשְׁאַר בָּהֶם עַד אֶחָד: 

28: And the waters returned and covered the chariots and the horsemen, the entire force of Pharaoh coming after them into the sea; not even one of them survived.

 The ibn Ezra brings our verse as a proof that just as here, "ad echad," means every single animal died,  so too, "ad echad," means every single Egyptian died (and not like chazal, who say pharaoh remained alive).( He brings additional support for this from the pasuk in tehillim, "וַיְכַסּוּ-מַיִם צָרֵיהֶם;    אֶחָד מֵהֶם, לֹא נוֹתָר. And the waters covered their adversaries; there was not one of them left," which clearly states that  no one  stayed alive, as further proof.

The Kehillas Yitzchak notes a difference between verses 6 and 7. In the first pasuk, the Torah says not one animal died from, "bnei yisrael." In the next pasuk, when in regard from pharaohs point of view it uses the term, "yisrael."

The Midrash in parshas Vayikra tells us about  a Jewish woman named Shlomis Bas Divri, who due to being overly flirty, was force to bear a child from an Egyptian father. Her name stems from the word, "dever," for she brought plague upon her son. Simply this means that she brought him into the world as the son of an Egyptain, and he pronounced the name of Hashem with a curse, and so she watched him be stoned.

The Ramban notes that before Matan Torah, Judaism was paternally hereditary. Therefore Ben Shlomis was not technically Jewish. After Matan Torah, as is the halachah today, a man born to a Jewish mother but gentile father  is  considered Jewish. 

So in verse 6, all the livestock of bnei yisrael was spared. The cattle of Ben Shlomis, who was not a ben yisrael,  did die. But to Pharaoh, in verse 7, he was still living among the Jews, and therefore he considered him to be Jewish. All but one cattle of the, "Jews" survived. Since at least a piece of the, "Jews," were killed, Pharaoh did not feel pressure to release them from bondage. 

This also sheds light on the meaning of, "she brought a plague upon her son." For if his father had been Jewish, he would have been spared from the plague of dever.

Have a Good Shabbos.