When you thread you are removing a TON of hair...lots of hair that you probably cannot even see. Every person, no matter how clean is COVERED in bacteria of various kinds including Staph. It's gross but true....so the more hairs you remove, the higher the statistical probability you could have problems. And remember that any time you pluck a hair, you could potentially damage the follicle and the hair could come in coarser and darker....it's not highly likely but it is possible. Just an FYI