Sounds like you've got a pretty good idea of how fish diseases work.
Fish that get sick after quarantine are usually exposed to excessive nitrogen amounts, which stresses them, which weakens their immune systems.
There aren't a lot of diseases that can hop from fish to inverts, but they exist.
Correct on the meds.
Often, one fish will be more stressed than the others, and will get sick first. If you do water changes (many small ones in a
SW tank so you don't accidentally change the
salinity), identify the source of the stress, figure out how to eliminate that stress, and treat the ill fish, the others usually have a chance.
There are, however, some diseases that are really virulent, and once their in your tank, they can be devastating (there's a freshwater one that affects labyrinth fish. If your fish is exposed, things are pretty grim from the outset).