There likely are; invertebrates in general (specifically molluscs and arthropods) typically don't go through the same level of disease diagnosis/treatment/study that larger macrofauna do, but are still host to a variety of viruses/bacterial/protozoan infection. In addition, gastropods are typically intermediate hosts in the life cycle of a parasite or other pathogen and so their effect is likely less visible. Finally, the life cycle of many gastropods and arthropods influences the progression, stage and visibility of disease.
This is not to mention that plenty of disease management occurs in the seafood trade, i.e. transmissable snail parasites affecting scallop populations, etc etc.
It's more likely that the amount of research done into pathogens is comparatively smaller and less visible and so we pay it no mind.
When you see a dead snail, do you know for SURE what killed it?