My standard line is that there is no such thing as a zero-risk animal. That’s not an anti-pet sentiment, it’s just life. The same thing actually applies to people too – there’s no person that’s not carrying something in or on them that could infect someone else. That’s not a reason to lock yourself in your room, just a realization that life has risks.
Back to crabs. As far as pets go, hermit crabs can be interesting yet low-maintenance little critters, and they’re easy to get. Some of the big concerns we have with other species, like bites and scratches, don’t really apply. You can get nipped, but it would rarely break the skin or cause any other problem. So, the question is whether they can carry microbes in or on them, or in their environment, which could cause problems for people.
As mentioned above, the answer can’t really be no. However, it’s probably as close to “no” as you can get with any pet (other than a pet rock). I can’t find any reports of diseases linked to hermit crabs. Depending how they are raised and distributed, I suspect there’s still some risk of them picking up potentially concerning bacteria like Salmonella, but it’s probably uncommon. Terrestrial hermit crabs live in moist environments that could harbour a range of bacteria and fungi, most of which are of very little concern, particularly to people with healthy immune systems. It would be similar with aquatic hermit crabs. There is the potential for various waterborne bacteria (e.g. Mycobacterium marinum) to be present in the aquarium, but the overall risk is low.
Overall, it’s probably at least as risky to have contact with a garden or sandbox.
However, a few common sense measures can be used, especially in households or facilities with higher-risk people (very young, very old, pregnant, or immunocompromised):
- Don’t clean out habitats or their contents (e.g. food bowls) in kitchen or bathroom sinks.
- Don’t dump aquarium water down the kitchen or bathroom sink, or bathtub.
- Clean up splashes and spills promptly.
- Wash your hands after contact with hermit crabs or their environment.
Are these precautions necessary? Maybe, maybe not.
Are they hard or disruptive? No. So it makes sense to do them anyway.