According to a controlled study by Dr. Robert Thiel, the group that used the Hulda Clark Zapper showed that the "total with any improvement was 97.9%" and "48.2% reported substantial symptomatic improvement in the zapper group". When compared to the control group who didn't use a zapper, "total with any improvement was 61.3%" and only "12.9% reported significant improvement". This clearly shows that the zapper had a positive effect in a study with a control group. There has never been any zapper study to refute this in any way. The main reason is that the zappers ability to kill many different microbes is a natural function.