"Inside John Barth" by William W. Stuart, also the answer to this old question and this one. Available at Project Gutenberg and the Internet Archive.
A man is injured in a explosion
No explosion, but he is injured. He's on a hunting trip with his Uncle John:
Then, all of a sudden, there was a bright flash of blue-green light and a loud sort of a zoop-zing sound. And a sharp, stinging sensation in my thighs.
I hollered. I jumped to my feet. I looked down, and my pants were peppered with about a dozen little holes, like buckshot. I didn't have to drop my pants to know my legs were, too. I could feel it. And blood started to ooze.
and later he realizes that he has become the host of a whole civilization of aliens.
According to the Official History I was given, they came from a tiny planet of a small sun. Actually, their sun was itself a planet, still incandescent, distant, perhaps like Jupiter from the true sun. Their planet or moon was tiny, wet, and warm. And the temperature was constant.
These conditions, naturally, governed their development—and, eventually, mine.
Of course they were very small, about the size of a dysentery amoeba. The individual life span was short as compared to ours, but the accelerated pace of their lives balanced it out. In the beginning, something like four of our days was a lifetime. So they lived, grew, developed, evolved. They learned to communicate. They became civilized—far more so that we have, according to them. And I guess that was true. They were even able to extend their life span to something like two months.
"And to what," I inquired—but without much fire, I'm afraid; I was losing fight—"to what am I indebted for this intrusion?"
It was, to them. Their sun had begun to cool. It was their eviction notice.
The aliens help him repair himself physically
"Invaded! Good Lord, of all the people in the world, why me? Nothing like this ever happened to anyone. Why did I have to be picked to be a territory—the first man to have queer things living in me?"
"Oh, please, gracious Fatherland! Permit us to correct you. In the day of our fathers, conditions were, we can assure you, chaotic. Many horrible things lived here. Wild beasts and plant growths of the most vicious types were everywhere."
"What you would call microbes. Bacteria. Fungi. Viruses. Terrible devouring wild creatures everywhere. You were a howling wilderness. Of course, we have cleaned those things up now. Today you are civilized—a fine, healthy individual of your species—and our revered Fatherland. Surely you have noticed the vast improvement in your condition?"
and provide him with help and advice.
All at once, almost anything I undertook to do was sensationally successful. I wrote, in several different styles and fields and under a number of different names; I was terrific. My painting was the talk of the art world. "Superb," said the critics. "An astonishing otherworldly quality." How right they were—even if they didn't know why. I patented a few little inventions, just for fun; and I invested. The money poured in so fast I couldn't count it. I hired people to count it, and to help guide it through the tax loopholes—although I was able to give them a few sneaky little ideas that even our sharpest tax lawyers hadn't worked out.
The one thing that they will not let him do is shave with a blade because a nick would cause thousands of alien deaths so he must use an electric razor.
I had always used an ordinary safety razor—nicked myself not more than average. It seemed O.K. to me. Never cared too much for electric razors; it didn't seem to me they shaved as close. But—I took to using an electric razor now, because I had to.
[. . . .]
I had to be careful because, as they explained it, even a small nick with a razor might wipe out an entire suburban family.
The man eventually finds a woman who is also infected with the same kind of aliens.
It was late afternoon, turning to dusk. She lifted up on one elbow and half turned away from me to switch on the bedside lamp. The light came on and I looked down at her, lovingly, admiringly. Idly, I started to ask her, "How did you get those little scars on your leg there and . . . those little scars? Like buckshot? Julia! Once, along about ten years ago—you must have been a little girl then—in the mountains—sure. You were hit by a meteor, weren't you?"
She turned and stared at me. I pointed at my own little pockmark scars.
"A meteor—about ten years ago?"
"I knew it. You were."
"Some damn fool, crazy hunter," was what Pop said. He thought it really was buckshot. So did I, at first. We all did. Of course, about six months later I found out what it was but we—my little people and I—agreed there was no sense in my telling anyone. But you know."
It was the other ship. There were two in this sector, each controlled to colonize a person. My own group always hoped and believed the other ship might have landed safely. And now they knew.
They go on a bender with alcohol and drugs resulting in the death of the woman but the man barely recovers.
"Don't ask," said the doctor. I wasn't going to. I didn't even care where I was, but he told me anyway, "You are in the South Side Hospital, Mr. Barth. You will be all right—which is a wonder, considering. Remarkable stamina! Please tell me, Mr. Barth, what kind of lunatic suicide pact was that?"
"Yes, Mr. Barth. Why couldn't you have settled for just one simple poison, hm-m? The lab has been swearing at you all day."
"Yes. At what we pumped from your stomach. And found in the girl's. Liquor, lots of that—but then, why aspirin? Barbiturates we expect. Roach pellets are not unusual. But aureomycin? Tranquilizers? Bufferin? Vitamin B complex, vitamin C—and, finally, half a dozen highly questionable contraceptive pills? Good Lord, man!"
"It was an accident. The girl—Julia—?"
"You are lucky. She wasn't."
"Yes, Mr. Barth. She is dead."
He finds out to his regret that all the aliens inside of him have been killed due to the alcohol and drug fueled episode.
They were gone! At last, after all these years, they were gone. I was free again, truly free. It was glorious to be free—wasn't it?
[. . . .]
But I feel awful.
Well—how do you suppose New England would feel today if suddenly all of its inhabitants died?
This was a novella or short story from the late 60's or early 70's.
A novelette, it was first published in Galaxy Magazine, June 1960.