I don't know of any human parasites that are carried by land snails,
nor can I imagine what the mode of transmission from snail to human might
be, unless you ate them raw.  However, fresh water snails are the carriers
of one of the most serious human parasitic diseases, schistosomiasis.  Three
different species of schistosomes, or blood flukes, infect humans, and they
are a major public health problem in many parts of the world, especially
areas with poor sanitation.  Snails become infected when the water they live
in is contaminated by human excrement.  The parasite passes through larval
stages in the body of the snail, culminating in a final larval stage that
leaves the snail, and is free-swimming in the water.  This larva is capable
of burrowing directly into human skin, and humans become infected when they
enter the water.  Once in the human body, the larvae migrate to the blood
stream, where they mature into full-grown blood flukes.  Each of the three
species of human schistosomes uses a different family of fresh water snails
as its intermediate host.  I don't remember what they all are - I believe
one of them uses snails in the family Hydrobiidae.  Schistosomes are flukes
- flatworms - but unlike most flatworms, they are not flat.  They are
slender and cylindrical, an adaptation for living in blood vessels.  Other
species of schistosomes infect other mammals, and birds, and each
schistosome species has particular fresh water snails that serve as its
host, and cannot use other snail species.  Sometimes the free-swimming
larvae of waterbird schistosomes will burrow into human skin.  They can't
infect humans, but  die in the skin, causing an irritating alergic condition
know as "swimmer's itch".  That's all I can remember.  If you want more
information on this, I'll get out my parasitology books when I go home
tonight.
        Regards,
        Paul M.