Most parasites, at least over 300 species are microscopic, so though you may not see or feel them, most people have them in some shape or form.
The humaworm does not only kill parasites but does a full body cleanse (in my opinion-I had a brown spot on my face that was wiped out by the cleanse that I thought could be cancerous but never went in to get it checked out because I was scared), and can target some Candida as well. It did get some of mine, the second one really got my candida and we are doing a third one here in May.
So lets get down to if you don't have parasites (which you probably do...we all do), if you do do the cleanse and you do in fact have no parasites you may not get sick. However, if your bum is itching, you probably have parasites, and you will feel a little wonky as they die. On humaworms website, there is a HUGE list of symptoms of parasites on there, as well as symptoms of die off as you are taking it.
It is something to definitely be mentally prepared for, so do some research. Our family is a loyal costumer of Humaworm. Our first cleanse was very Sci Fi stuff! It is gentle yet efficient. Probably slower then some cleanses, but we like it.
Always feel free to contact them to ask your questions! They are really sweet!