Originally Posted by
Kimera757
The carapace thing is just his armor upgrade, which all ground zerg units have.
Personally I think one of three things happened:
1) He had a parasite in him, the same type Starry Lace had, which he got on Tarsonis. Later, Kerrigan "bit" and infested him. She would have had time to do that between UED missions 7 and 8. (I think this one is the least likely, not least because the idea of the mind-control parasite hadn't existed yet.)
2) He was actually infested by the Overmind, but was such a good psychic/biologist using xel'naga secrets that he only got the positives, not the negatives. As a good psychic, he was able to "filter" what Kerrigan could read from him, kind of like how Ethan Stewart though non-suspicious thoughts (for a crime lord) to prevent Jake Ramsey from reading his real aims.
3) As above, but was faking it. He first appeared to Kerrigan as an "infested" terran.