Awesome! Glad to have this added to the conversation.
I actually had this thought and was thinking about adding something like this earlier today.
You’re technically correct, in a sense. There still needs to be lots of these to cause problems. If there aren’t lots, there’s no problem.
It would be the same for any self replicating thing. Bacteria, viruses, fungi, and prions, but they replicate. I will grant you a single large parasite could do this, but at that point, we’re talking about tigers and such as a technicality as well.
Potentially one of these things could cause problems by reproducing. I think it’s just unlikely. I don’t know how we could demonstrate that though. I imagine a single virus or bacterium can lead to disease. I just suspect the probability is low.
Like you, my first thought was prions, but they have to actually come into contact with the protein to catalyze its misfolding. That’d be rare in the protein soup, I suppose.