Our DNA carries viruses in its composition! A rather uncommon alliance, initiated not by our organism. The fact is that viruses use our DNA to reproduce - it helps them to copy (as DNA does). And in the case of a virus inserting its DNA into a reproductive cell - the child can get that DNA with the virus. And this process has been going on for far more than the first generation. This is what has led to the fact that now at least 9% of our genome are viral genes.