An early novel by Greg Bear
about a biologist
whose ethics slip in favor of the project he's working on. After being fired from his job for unauthorized experimentation
on human cells (his own), the scientist sneaks his work out of the lab by injecting
it into himself, with the intention of rebuilding his experiment from cultured cells re-donated by drawing his own blood
Things start to get odd when he notices changes in his skin and, soon, his entire body, as the little buggers start working on him to "improve" his body. Then they start talking to him.
This is a great novel and a science fiction classic. My personal favorite from Bear. It starts the tradition of ending the world that Bear follows in at least three other novels, but does so in a more interesting manner than The Forge of God.