Judging solely from the picture (meaning there's a fairly good chance that I'm wrong), this looks similar to Neon Tetra Disease. My rasboras get something similar, though it's far slower-acting than normal NTD and only affects one or two at a time, while also taking a good two to three weeks to actually kill the fish.
This is based solely on the appearance of the cyst on the fish, so as I said before, there's a large margin of error in my guess.