Would it be possible that the zapper's parasite-killing
effect (or part of the effect) is from the chlorine?
Human blood is mostly salt water, and electrolysis
(passing a DC current through salt water) creates
chlorine that will disolve in the water/blood.
Chlorine can kill parasites/bacteria/virus.
If the amount of chlorine is small human may
not be harmed.
If chlorine is used this way to "disinfect our blood"
(like disinfecting the swimming pool) maybe some
side effect can be contributed to human response
to chlorine, not just die offs.
Does this make sense ? (I am not sure, I am not an
expert in electrolysis)