Parasite Achievement has nothing to do with the srill / symbiont augment.











Actually the Parasite achievement is related to a random event. It is one of the anomalies you can find in space. Those small things marked by a danger sign on the star map:



They contain some random event like discovering a damaged pod (new crew member), finding valuable ore on an asteroid, a disease killing a crew member etc. And sometimes it's the "Brain Slug" event. If you run into this anomaly you automatically get it. You don't even have the possibility to make a decision.



As a side note: I don't see that anything changed to a crew member. The event doesn't seem to have any effect at all. Not even a note in the crew members augments or stats.