It has been shown that much of the DNA in our cells is a result of retroviruses.  To boot, HIV is a retrovirus.  A retrovirus invades the cell and becomes part of an individual's genome, and generations pass them down through vertical gene transfer.  The way a person picks up a virus or retrovirus is through horizontal gene transfer.  HGT is, simply put, the process of individual contracting DNA from it's environment. 

So, we have ancient virus' within all of us.

Here is a link describing retrovirus' and our genome: http://www.pubmedcentral.nih.gov/articlerender.fcgi?&pubmedid=16611924

Here is a link describing vertical and     :

http://www.physorg.com/news114185292.html

I hope this finds you curious!