Some things like some bacteria, algae, some fungal spores etc are a part of the natural environment everywhere. They may be fine at low or usual levels but are waiting for a chance to spread if the right conditions are met. Stressed or weakened fish, a swing in water parameters, it could be anything that allows whatever to get a big foothold.
Fish & live food can often carry a low level parasite load that doesn't necessarily cause trouble but can if a new fish or very different parameters allow for their spread. Some "critters" are harmless & may be a form of live fish food. I have a many years old colony of blackworms in 1 tank. My fish may eat some but after all this time I'm sure they're fine. I also see stylaria worms once in a while, no big deal but I don't know if I have now fish that will eat them.
& of course anytime you introduce new animals or plants there's a chance they come with "something". There isn't a realistic way to prevent all chances of introducing "things". But I strongly suggest a quarantine tank for animals & at least a chlorinated tap water soak plants & "used" decor. I never allow "other" tank water into my tanks as best I can.
