Even though the Bnai Yisrael were not subject to any of the plagues, there is evidence that during the plague of lice, even the dust in Goshen was affected. Rabbi Sorotzkin in Oznayim LaTorah explains this in Vaeirah 8:13 based on Bereishis Rabbah, chapter 96. Yaakov Avinu asked that he not be buried in Egypt and insisted on being taken to Mearas Hamachpeilah to be buried with the other Patriarchs and with the Matriarchs. One of the reasons given is that the dust of Egypt was going to become infested with lice during the coming plague. It appears from the way it is presented, that the dust was infested by the lice so that the Egyptians could not run to Goshen for safety.
Mishnas Rabbi Eliezer (chapter 9) states that as a result of this plague, the Bnai Yisrael no longer had to make bricks, because there was no longer any clean earth to use as the raw material. Had the lice been kept out of Goshen, Pharoah would have just moved the brick making factories there.
Another reason is that Pharoah was able to use this as an excuse to refuse to believe that the Bnai Yisrael were not affected by the plague. He was able to insist that since the lice appeared in the area, everyone living there must have been affected.
Another point about this plague is that the Priests and Nobility were phobic about lice. They were the first group to shave on a regular basis. The priests, for example, shaved off all of their body hair. When the plague occurred, they paniced immediately.