Actually, thousands of mis-diagnosed syphilis patients were released. You said it yourself, the great imitator. The chances of making such a diagnostic error these days is very small because it is so well known.
As for comparing zappers and ECT's, they are quite different. Most ECT's are current-mode devices. They adjust the output voltage (to as high as 450V) to maintain a regulated current through the brain. Zappers, on the other hand, are constant-voltage devices (ok, PZ, *most* zappers...). The total energy pumped through the brain in an ECT session is about 1000 times greater than that of a zapper session. Plus, the fundamental theory of what is happening is very different. An ECT session basically forces the brain to "re-boot" by scrambling the patterns of electrical signals. An analogy would be the intense electromagnetic field of a lightening strike down the street forcing your PC to cold boot. The ECT pulses are designed to maximize current flow while keeping brain heating to a safe (??) level. Nothing even remotely close to the targeted frequency theory behind zapping, Rife, etc.
Still, almost anything is possible. I'm checking with some psycho-buddies about the viral causation.