However, the exact influence that viruses have on host DNA is not well known, yet speculated that a large majority of DNA and even mutation could be the result of virus interaction, not just in bacteria and single celled organisms, but indeed in higher organisms.

There was and still is often talk of JUNK DNA and the like, but this is not strictly true, as science advances we learn more and more.

There is a lot of DNA in many animals that appears to do nothing, sometimes as much as 95%, but it has been proven to act in other ways such as redundancy, protection from fatal mutations.

You should read up on mobile genetic elements, introns exons, regulation of protein coding sequences and pseudogenes.