Blood Music by Greg Bear

Blood Music by Greg Bear1 (Analog, June 1983) opens (after a short and essentially irrelevant passage) with a doctor called Edward meeting an old university friend called Vergil, an odd-ball whiz kid who, among other japes, “wired door knobs, [and] gave us punch that turned our piss blue”. After some social chit-chat, and discussion of some of Vergil’s changed physical characteristics (he’s fitter and more tanned), Edward learns that his friend has been working for a company called Genetron developing medical microchips. Edward also learns that Vergil was fired, but has been continuing his research outside the lab. Virgil then tells Edward he wants him to put him through a thorough physical exam.
When Edward conducts the examination, he finds that Vergil has a lot of very odd physical characteristics:

“Look at my spine,” he said. I rotated the image in the video frame. Buckminster Fuller, I thought. It was fantastic. A cage of triangular projections, all interlocking in ways I couldn’t begin to follow, much less understand. I reached around and tried to feel his spine with my fingers. He lifted his arms and looked off at the ceiling.
“I can’t find it,” I said. “It’s all smooth back there.” I let go of him and looked at his chest, then prodded his ribs. They were sheathed in something tough and flexible. The harder I pressed, the tougher it became. Then I noticed another change.
“Hey,” I said. “You don’t have nipples.” There were tiny pigment patches, but no nipple formations at all.
“See?” Vergil asked, shrugging on the white robe. “I’m being rebuilt from the inside out.”

Vergil explains that the changes are a result of his work with Genetron which, essentially, was to do with designing nano-biotechnology (although this phrase isn’t used). He explains how he injected the company’s smart proteins into bacteria, which could then repair themselves, compare memories, and evolve:

“By God, you should have seen some of the cultures a week later! It was amazing. They were evolving all on their own, like little cities. I destroyed them all. I think one of the Petri dishes would have grown legs and walked out of the incubator if I’d kept feeding it.”

So far, so Microcosmic God,2 and Vergil goes on to explain that, by the time he exponentially improved his cell cultures, the company had discovered what he was doing and forced him to destroy his work. Before that Vergil injected himself with some of his own altered white blood cells, and they have since been modifying his body. Vergil then tells Edward he is worried that the cells will eventually cross the blood-brain barrier and “find him”—so he wants them destroyed.
The rest of the story (spoiler) sees Edward run more tests but, by the time visits Vergil a few days later, his friend says he can hear the cells talking to him—blood music”. By this time they know who he is, that they are inside his body, and they are trying to understand the concept of space. On a later visit Edward finds out that Vergil has been examined a second time by a Dr Bernard, an associate of Vergil’s old company, and also that Vergil’s physical changes have become more pronounced. Edwards asks Vergil to tell the cells to slow down the changes:

“You’re . . . you can talk to them, tell them to slow down,” I said, aware how ridiculous that sounded.
“Yes, indeed I can, but they don’t necessarily listen.”
“I thought you were their god or something.”
“The ones hooked up to my neurons aren’t the big wheels. They’re researchers, or at least serve the same function. They know I’m here, what I am, but that doesn’t mean they’ve convinced the upper levels of the hierarchy.”
“They’re arguing?”
“Something like that. It’s not all that bad. If the lab is reopened, I have a home, a place to work.” He glanced out the window, as if looking for someone. “I don’t have anything left but them. They aren’t afraid, Edward. I’ve never felt so close to anything before.” Again the beatific smile. “I’m responsible for them. Mother to them all.”

Edward thinks Vergil is more of a host than a mother (or “super-mother” as Vergil later refers to himself) and arranges to meet Dr Bernard to see if he can help.
When Edward next visits Vergil he finds him sitting in a bath tinged pink with his blood—“astronauts” sent out by the cells to explore the exterior environment. When Vergil goes to pull the plug and release them the world, Edwards ends his agonising about the threat that Vergil poses (this dilemma has played out in parallel to the above in scenes where Edward has been sleeping—“Vergil Ulam is turning himself into a galaxy”—or with his wife), and he throws an electric sunlamp into the bath killing Virgil and the cells.
The last act of the story sees Edward go home. He and his wife subsequently fall ill, and Edward deduces that Dr Bernard infected him (from the damp handshake he received). The white cells take over Edward and his wife’s bodies, communicate with them, and then meld the pair together biologically. The organism created then grows to fill the apartment, and spreads out beyond it: mankind is doomed.
This is a very good piece of work which manages a tour de force combination of several SF tropes including scientist-as-God/messiah, alien body horror, the end of mankind, and, ultimately, the Fermi Paradox (why is there no sign of other intelligent life in the Universe?) The last two transform the story from one that begins on a microscopic level to one that eventually has cosmic implications.
****+ (Very good to Excellent). 8,750 words.

1. This was expanded into a novel of the same name published in 1985.

2. Microcosmic God by Theodore Sturgeon (Astounding Science-Fiction, April 1941) is reviewed here.