A question was posed to Maran Rav Chaim zatzal regarding Lag BaOmer:
What exactly is the joy that on Lag BaOmer the students of Rabbi Akiva ceased dying? They were all gone – there were no more left! That’s why they stopped dying.
Rav Chaim answered: Most of them died, but a minority were left. This is similar to what the Baalei Tosfos have written Bava Basra 121a “Yom” regarding the remnants of those who died in the midbar.
Further question: The Gemorah seems to indicate that the world was barren until Rabbi Akiva came along and went to the Rabbinic masters of the south – that there were none left.
Rav Chaim zatzal answered that of the important ones – none were left and that is why he travelled to the Rabbinic masters in the south. (See Kuntrus Likutei Halachos on Yevamos p. 25)