Taking this from the previous thread:
Let me explain it in terms you might more readily understand. Human DNA is no different from any other DNA. For example, an influenza virus contains DNA. If someone sneezes into their hand, and at some later points grabs something like a doorknob, virus can be transferred to the doorknob. suppose you then come along and use that doorknob. Now virus potentially is transferred to your hand. Suppose now that you use this hand to eat with, or wipe you nose (or whatever). Now that virus can gain entry to your body and infect it. This is how most flu infections happen and why you are supposed to wash your hands.
Transferring human DNA is exactly like that.
You do not need to know the person the original DNA came from, or even met them, all you need is a route of transmission, and in this case there is a very clear and obvious route.