Was reading
this interview with Terry Matalas, and he reveals how the parasites were indeed on the table for the villain this season. However they decided against it because:
"I do like the conspiracy bugs, I did consider for a hot minute, the conspiracy bugs. The issue with the conspiracy bugs is they kill their host. So that would mean anyone that you saw with a bug in them was dead, and you wouldn't be able to do the paranoia thriller that you wanted to do if it was somebody you loved, that meant they were dead."
Except, they don't.
Admiral Quinn was seen alive and well at the end of that episode, despite having been infested with one of the parasites.