I thought Darkness Falls was about the insect swarm (still a great episode, regardless) but the one I’m thinking about in particular was very early in the series, maybe the 7th or 8th episode, and there was a parasite buried in some ice somewhere, then a sample melted and it wound up in a border collie...
Well when you think of NASA scientists wanting to bring back micro-organisms from Mars for which we have no natural defense, someone needs to come along with a ruler and smack them across their hands.
Oh yes! I don’t think I ever saw that entire episode though.