Kristen,
My best shot in the dark guess would be a bacterial infection though I question if that is the case with this fish. Bacterial infections though usually show up in the form of white patches of increased slime coat or even fungus. In addition to that bacterial infections are usually the product of highly diminished water quality; if your fish has been healthy for four years I find it hard to believe that your water quality was bad or bad enough to cause the fish's demise.
This could have also been a parasitic infection though. The white patches could represent areas where the fish was glazing against rocks and other decor in an attempt to scratch off parasites. I don't see any external parasites on the fish though that does not mean that they are not there. That goes without mentioning the possibility of internal infection as well.
Sadly without getting tissue samples under a microscope with someone trained to identify various parasites/bacterias the cause of your fish's demise will likely be unknown. I will say in one photo the fish's color near the bottom looks pale and I can see what somewhat looks like marine velvet cysts. Marine velvet is much smaller in diameter then ich and can sometimes be hard to diagnose. Sadly this parasite is a true fish killer and can wipe fish out with much higher efficiency then marine ich; in short it's no fun!
Posting your current water parameters and an idea about what this fish was fed would be helpful in determining what the problem might have been. As for you question about bacterial infections: They can transfer fish to fish though like I said earlier they are normally the cause of diminished water quality or sudden temp spikes. Does you tank have a
UV sterilizer on it.
*EDIT*
Kristen,
I enlarged the one picture of your fish and carefully examined it. I can see what looks like a mixture of marine ich and perhaps other external parasites all along the fish. I would assume the patches you see are from stress compiled with other factors caused by the intense parasitic infection. In my opinion it was most certainly an external parasite that killed your fish and I would consider some options as your other tank mates may be infected.