Rashi on Numbers 21:6/רש"י על במדבר כ״א:ו׳

1את הנחשים השרפים BURNING SERPENTS — so termed because they burn (cause fever in) a person through the poison of their fangs.

2וינשכו את העם AND THEY BIT THE PEOPLE — God said, as it were: Let the serpent which was punished for slanderous statements come and exact punishment from those who utter slander. — Let the serpent to which all kinds of food have one taste (that of earth; cf. Genesis 3:14 and Yoma 75a) come and exact punishment from these ingrates to whom one thing (the manna) had the taste of many different dainties (see Rashi 11:8) (Midrash Tanchuma, Chukat 19).