It takes a lot of natural talent. Let me explain what natural talent is. Natural talent is a combination of speed and endurance. The speed for the 800 can be trained to a certain extent, and especially with the endurance. However, there is always, always a baseline that someone starts from. Meaning that if someone starts at 2:00 minute 800 meter with no training, they have a chance of breaking 1:48 with training. Now on the other hand, someone that starts from 2:20 with no training has NO chance of breaking 1:50, probably not even the 2 minute mark.
The 800 meter requires a combination of speed , you must be about to run about 50 second 400 if you want to break 1:48. A 50 second 400, while nothing special, is the minimum speed required to break 1:48. And this 50 second 400 would be someone like a 1500 runner who occasionally runs the 800.
A true 1:48 800 runner (who trains for the 800 ), has around 48-49 speed for a 400.
I am assuming the person is also not overweight and fully grown.