Yeah, I definitely didn't explain the concept well enough in the tutorial, so I can understand the confusion!
The idea is that a parasite takes control of one of the characters and forces them to kill the other survivors in the shelter. You first have to figure out the parasite's identity by looking at the pattern of the victims' deaths. Then, among the survivors, you have to determine who is infected by looking at their symptoms.
All characters can have random symptoms, but only the character carrying the parasite will have both symptoms associated with that parasite.
So if you were trying to match a single symptom to the parasite, that would definitely make the game seem broken 😅. Thanks for the feedback! I really should have explained this mechanic much more clearly in the tutorial.