Can snails give fish disease?

[LDZ]

A month ago I got a small Trapdoor Snail.  I added it to my Betta tank.  I didn't quarantine it because I read snails don't carry fish diseases now thinking that I can get might be wrong.  Ammonia 0 nitrites 0 ph 7 well cycled tank is.   About two weeks ago my Betta started to show symptoms of bloat, swollen belly, and extremely lethargic.   He use to be extremely active and now all he does is sit on the bottom of the tank.  I fasted him and have been feeding daphnia and spiralina for a week.   He has a really good appetite and still eats really well.  Could the snail have given him anything???  I also have a few neo shrimp with him.  FYI I got him from a lps a year ago and he is a dragon scale with diamond eye and completely blind.  I knew it when I got him.  I taught him to eat in a feeding ring. He has also had multiple small tumors coming on over the last few months.   What should I do.  I'm reluctant to move him to a quarantine tank because of him being blind.   I have also considered he could be getting old.   The thing I don't understand is why is his appetite so good for him acting so sick?   Thank you for your help!!!

[Guppysnail]

Yes and no. Snails do not harbor diseases in their body that transfer to fish. However the water on/in their shell and body can contain things undesirable. They can be used as an intermediary host for some parasites that if the fish eats the snail the fish them become a host for the next stage of the parasites life cycle. 
 

There are so many causes of the symptoms you are describing I would be hesitant to blame the snail. It could be the stress of getting to know a new roommate tipped the scales in favor of some previously unseen illness the betta was was dealing with. 
 

Water quality long term  (high nitrates) are always suspect but so are many other things. I’ll let the disease advise folks comment on treatment.