All four of my dogs and my cat are from the shelter. i also have fostered many...they've all been fairly healthy. the foster pups have had their bouts with worms and URIs thats why we pull them out of the shelter and into foster care, sort of preventative and also to treat their minor ailments before they do get severe. we've been very fortunate. i know of other animals that have been placed in foster care only to die of some illness, whether it's parvo or a terrible case of heartworms. shelter animals arent all sickly, but you do run the risk when you adopt from a shelter.
and as for ear cropping...here's my opinion of it...
this is Luchi, my "failed" foster. he's a black/white pit bull. i fostered him for a few months and finally adopted him in September. someone decided to go cheap and crop his ears themselves. after they botched the job, they dumped him at the shelter when he was 3 months old. we do wonder if they tried to do a battle crop. it's common here, people fight dogs. and they crop the ears down to the scalp on the bait and fight dogs. very sad.
Ear cropping isnt necessary. it's usually done for cosmetic reasons. and most people ask me about cropping the ears of the bulldogs i foster. i dont recommend it. too much of a risk of infection or the job getting botched. it takes a lot of skill to crop an ear correctly.