Thanks Bear :)
redchigh, you might want to read this then, a quote from the article:
"Ilyanassa is an intermediate host for at least nine species of trematode fluke. In most populations, over 50% of the adult snails have at least one species of this parasite, and in some populations the rate of infection is as high as 94% (Curtis and Hubbard, 1990; Curtis, 1997)! While none of these flukes actually targets humans as hosts, they do sometimes mistakenly burrow into human skin and cause what is known as "swimmers' itch," or more properly, cercarial dermatitis (Sindermann, 1960). This condition creates an extremely itchy rash similar to poison ivy that can last for up to a week. Affected snails will continue to release the infective stage of these flukes into a tank for up to a decade (assuming the snails live that long), and any exposure to the tank's water puts the aquarist at risk of infection (Curtis et al., 2000). Interestingly enough, the flukes that infect Ilyanassa harm them not only by stealing nutrition and castrating the animals, they can also actually control the snails' behavior. Infected snails make more frequent trips to, and go higher into, the intertidal zone where they are more likely to encounter birds and crustaceans, which are the flukes' primary hosts (Curtis, 1993). Coincidentally, that's also where they are more likely to encounter livestock collectors. As a result of this behavioral modification, it's highly likely that collectors of these snails have an unintentional bias toward infected specimens."