how does the zapper work internally

Please explain how the zapper works. It puts electric current through your body? how is that safe? I know it is a small current, but how does it only kill parasites? What about our red blood cells for example? Does it cause any damage to our organs?
I just don't get how it can kill the live bad guys but not the live good things like cells.
And where do you put it, on your belly?

I know people who go on chemo are told that the chemo may kill the cancerous cells, but it also kills healthy cells. That was my first thought.
If this is so cheap (10 bucks is nothing!) then why doesn't everyone get one?
I am not trying to be cyncial, jsut trying to understand all the info. Tweet