In fact (though I can't quote a source for this), humans have so many commensals, not just parasites, that the majority of DNA in the human body is not the DNA in the nuclei of your cells. Gut flora, famously, assist us in our digestive processes, which is one of the reasons why anti-biotics, being slightly indiscriminate in their actions, result in uncomfortable side effects - they decimate (literally take a tenth?) the gut flora as well.