Mika's right, to the best of my understanding. . .
I'm not an expert by any means, but the only thing that comes immediately to mind that can *potentially* be carried by these snails and affect humans are certain trematode parasites, that can cause the condition we call 'swimmer's itch.' Swimmer’s Itch typically presents as itchy red 'pinpoints' on the skin that *typically* lasts for about 2 days - up to a weekish, and usually will clear up without any medical assistance. I've seen this in person (nothing to do with tanks or snails!) and it usually is fairly harmless. Is this what you're concerned about?
When looking at any parasite, it’s important to keep in mind that each type has their own series of unique requirements that often involve a range of various hosts to complete their specific life cycles, as Mika pointed out. Because of this, many of the parasites that can potentially affect humans are not commonly found in our aquariums, as the chances of the required hosts being found in one tank/home are fairly slim. Trematode parasites are a huge group of buggies, and there are only a few in this group that can cause Swimmer’s Itch.
If I’m remembering correctly, these are members of the Schistosomatidae family, and typically the ones that we’re concerned with will require a bird (some need rodents) to complete their full life cycle and reproduce. During one phase of their development, these creatures become free-swimming in the water (in search of their next host), and this is when they are able to affect humans. Though they will burrow into human skin (which triggers our immune system to create the associated rash), it’s a mistake on their part. Humans are NOT able to act as a host for these parasites, and so they will ultimately die without reproducing (unless they are somehow able to find a duck or a rat in your tank). Since they can not reproduce, the infection (if present) will be short-lived, and all parasites will die-off and your snails will be ‘clean.’ If you’re terribly worried about the rash, using gloves to protect your hands while doing maintenance on the snail tank should protect you from any possible issues. :)
That said, you will often find these snails associated with other, nastier types of Trematode parasites that can cause real harm to humans (don’t freak out yet). These parasites can cause a range of chronic illnesses in humans, because we are a viable host for them, and so they would be able to reproduce and complete their life cycle. Each have specific names, depending on the particular parasite that is the cause, but Schistomiasis, or Snail Fever is often used to reference all of them. This is a huge problem in Africa, and other "developing countries". . . I truly have no idea if it is a major concern in Iran, but I would look further into it - most especially if these snails were wild caught locally, as there may have been infected non-Apple/Mystery snails in the water where they were captured that *could* affect you.
The good news for US is that although they can infect Apple/Mystery snails, these types of snail are NOT a viable host for the parasite, and so they are unable to complete their life cycle and reproduce to infect humans. In fact, Apple Snails are often introduced in areas where these parasites are a problem to help control the spread of the illness! Not only do Apples keep the parasite from spreading (even if they are infected), they also prey on those snails (and their eggs) that are proper hosts, as well as taking their food sources for their own - all of this effectively cuts down the numbers of potential hosts that will allow these parasites to thrive, and reducing the overall population of the parasite. . . interesting, but of no real concern in the home aquarium.
All that said, I’m still new to the world of tanks - including snails. This is all pulled from information that I’ve read over the last year or so, and it’s been a while since I’ve looked into things. Be sure to do your own research and double-check what I’ve said with reliable sources. I’d hate to see you become ill if there is something I haven’t learned about (and there is so, so much left for me to learn!) Hopefully someone with more experience in these things will be along soon to add their own experience and information to further help you out.
Last edited by Chesh; 04-16-2013 at 10:10 AM..