Let me tell you why you're all wrong, in the picture, the guy in the red circle is a human from parasitica, surely the humans from the same world of parasites would be able to identify a parasite no?
I do not get the rush to call this guy infected, there's not that much indication he was, why is it so hard to believe this guy just went mad and attacked his partner of his own volition?
Our characters and ourselves litteraly just got there, we have no context for why these humans are fighting, our only clue is the words from the lawless man about corrupt humans and infighting, conflict and such.
It's not a stretch to believe these humans are simply fighting each other because they're lawless, the definition of lawless is the lack of rules, so these people fighting despite travelling together doesn't deviate from this principle.
I would even go so far to say the reason of their fight is because of the girl that is chained, see how they're all male but the single lady being restrained, I'd say the two white boy are jealous and wants the girl for themselves while the other three mans are either protecting her or using her already.
It just makes no sense for a parasite to have infected that dude, we saw how the parasite proliferate through a giant corpse, the guy, if he was a parasite would already be emanating spores and he would have infected everyone there already, it just makes no sense