“Legacy” is episode four of season three of Stargate SG-1.
SG-1 go through the stargate to an ancient Goa’uld facility, filled with the dead bodies of Goa’uld hosts. It’s not known how they died, or whether they were killed, either directly or indirectly.
Upon returning to the SGC, Daniel starts hearing things, and then seeing things, primarily the dead Goa’uld hosts.
The team try to figure out how the Goa’uld died; unfortunately for Daniel, he sounds more crazy than anything else, and his doctors think he is suffering from schizophrenia, and that everyone else is going to be at risk.
Of course, that’s not going to be the right answer, is it? Nothing is ever that simple – otherwise, they’d be very short episodes.
The Goa’uld (the symbiote) definitely look a lot better (more realistic?) than they used to.