A research paper recently posted to Pastebin titled "Infection of biological DNA with digital Computer Code" claims a theoretical method which could be used by self-replicating malware to alter DNA.
The infection vector is somewhat similar to Stuxnet - infect computer systems then alter "fasta" files containing DNA information. When the altered files are synthesized, the computer code makes the transition into the biological realm as bacteria, where it continues to self replicate as a living organism.
I am by no means a microbiologist (and neither is the author of the paper), so I can't very easily confirm or deny the paper's contents. It sounds plausible, but then again it could have easily been created by modifying a paper generated by the CS Paper Generator.
The paper author is on twitter @SPTHvx.
Apparently a proof of concept virus is on SPTHvx's website.
Analysis of the proof of concept virus available at the following locations: