In several episodes remarks are made about the Jaffa. Sometimes to the effect that Jaffa are not human, sometimes hinting at some link between the two but usually implying that a Jaffa would not be able to become the host of the larval Goa'uld s/he is "incubating".
In episode "The Other Side" (S04/02) the leader of the Eurondans has a brief dialogue with Te'alc:
Alar: You are not of their kin?
Te'alc: I am in fact a Jaffa ... though like you our ancestors are descendants of the Tau'ri.
So if the Jaffa are descendants of ancient humans what makes them so different? Why can't they become Goa'uld hosts themselves? Has this been explained in SG-1 lore and I merely missed it?