Comentario sobre Números 21:7

ויתפלל משה AND MOSES PRAYED — From this we may learn that he of whom one seeks forgiveness should not be so cruel as not to forgive (cf. Midrash Tanchuma, Chukat 19).

התפלל אל השם, "pray to the merciful G'd, etc." In this instance the meaning of the word התפלל is equivalent to a call for forgiveness of the sin. There is a difference in the accessibility of forgiveness if one prays for it before the guilty party has experienced afflictions or if one has waited until after he has experienced afflictions. Once the guilty party has already become the victim of the destructive forces he himself has created and which have come home to roost, he needs to marshall some merit in order for these destructive forces to be called off. Shabbat 32 has this to say on the subject: "A person should make a point of asking for mercy before he falls sick because once he has fallen sick they say to him: 'produce some merit and you will be freed from the sickness.'" This is why the people of the generation of the desert (also known as the דור דעה, the generation blessed with knowledge) were astute enough to ask Moses to pray before asking him to remove the destructive serpents. They wanted Moses to invoke his own merits on their behalf.

הנחש, a collective term meaning many of them.