Hashem instructed Moshe not to count the Leviim ‘among the Jewish people.’ The Midrash explains that this was in order to save their lives from the angel of death. The expression ‘Seu Es Rosh’ foretells of the death of the entire Jewish generation in the desert.
Sources
Bereishis Perek 40, Pesukim 5 - 19
Rashi on Possuk 5
Bamidbar Perek 1 Possuk 2 and Pesukim 46 - 50.
Bamidbar Rabba Parsha Aleph Perek 11
Bereishis Rabba Parsha 88 Perek 5 - 6
Likkutei Sichos Volume 4, page 1021
