Hermit crabs don't usually get sick from any known diseases. They do get parasites. There are probably diseases among crabs that we don't yet know about. One of the leading causes of death among any hermit crab is molting. During this time, the crab is very vulnerable. Most usually die of dehydration. Crabs also easily die when using aerosol or other chemicals around their cage (such as hair spray, air freshener, bug spray, table or glass cleaner, etc.). Crabs can die from these, old age, parasites, or a lack of large enough shells for them to change into.

Most crabs die during molting. Since a crab has a hard outer shell, they can't grow like we do. They have to shed this hard layer, grow, and then re-harden a new shell. Smaller crabs molt quite frequently, since they grow a lot more. Older, larger crabs will only molt a few times a year. It's easy to tell when your crab is molting. Their activity will drop off severely, they'll dig into the sand to hide, and won't eat. They'll stay this way for a long time.

Molting is the only time when crabs grow back missing legs or claws. If they've completely lost the appendage, then it will probably take a few molts to completely re-grow.

During molting, crabs stay in their shell because they are very very vulnerable. Be sure to moisten them by spraying them, and then return them to their hiding place. Crabs don't eat much, but you might want to put some food next to them. After molting, the old skeleton will still be there. DON'T REMOVE IT FROM THE CAGE!! The crab will eat it for the calcium it contains.