Antihypoxiant by Andy Weir (roughly 1.300 words - it probably takes about 5 to 10 minutes to read the whole story)
The following citations are from the linked website. I just cite stuff that shows things similar to what I remembered. As this is a very short story I will cite quite a lot of the text:
Best start into a story:
I am, simply put, the most brilliant medical mind in human history.
He is a doctor:
But it can’t be helped; you’re not a doctor. Or if you are, you’re not one of my caliber.
He invented a cure for what he discovered is what really kills people:
Hypoxia is what kills people. Whatever the “cause of death” may be, the actual reason they die is because oxygen stops getting to their cells. Most notably, to their brain.
He did some experiments with rats:
I had a disembodied rat hamstring suffused with the compound, then I just let it sit for three days. By rights, it should have been completely dead, but upon examining it, I found that the cells were still very much alive and healthy.
Zombies (though it's not quite obvious at this point):
so long as there wasn’t physical damage to your brain, you won’t die for several days. [...] You can literally get shot in the heart and they will have enough time to perform a heart transplant to save your life.
He modified a virus:
The original virus was a harmless strain that usually causes mild vasculitis. It attacks the cells of arteries, veins, and capillaries. This was ideal, because it allowed my modified version to deliver the antihypoxiant to those cells.
He tested on animals:
I tested on cats, dogs, and eventually monkeys.
But he had a problem with authorities:
But the FDA was the next step. After animal trials comes human trials. First with a tiny control group, then with larger and larger groups and so on. Getting a drug through the FDA can take ten years and that simply wouldn’t do.
He was too impatient:
I was 57 years old at that point. Waiting another ten years would make me 67. Then, it would take at least another five to ten years before the Nobel committee recognized my genius and awarded me the prize I so richly deserve. There was a chance I wouldn’t live to reap my due fame! [...] No, the FDA was an impediment. It was time to act.
So he took the short path:
I released the virus in several major population areas all over the world.
There is a certain side effect he hasn't accounted for:
It turns out that the higher cognition centers of the brain have a slightly different blood-brain barrier setup. My compound, while transitioning into those parts of the brain, changes slightly due to the barrier and ends up malfunctioning.
Zombies (now quite obvious):
In any event, it caused people to slowly lose their higher cognition abilities. They retained motor control, sensory input, fight or flight reflex, etc. But they lost their ability to “think”.
Animal tests wouldn't have helped, no matter how much he tested:
Cats, dogs, and monkeys don’t have that part of the brain at all.
Great humor if you ask me:
Some could argue this is my fault, but that’s absurd. Clearly the FDA is at fault. If it weren’t for their slow process, I wouldn’t have been forced to distribute the compound like I did.