In the first two generations, you gained EVs equal to the base stats of the pokemon you defeat, so say I beat a typhlosion in battle. I get:
78 HP Evs
84 Attack EVs
78 Defense EVs
109 Special attack EVs
85 Special defense EVs
100 Speed Evs
You would then take the square root of however many EVs you have, then divide by four.
You could have a maximum of 65535 EVs for each stat, and vitamins would add 2560 EVs. Unlike the current system, where you can only distribute a certain number of EVs in your stats, you were able to max out all EVs in the earlier generations. They picked 65535 because the square root is equal to about 256, which divided by 4 equals 64, just like the current EV system. Due to the programming, and the specialty of the number 255, they have to round it down, meaning you get 63 points.
The pokerus did exist, but only in the second generation, still yielding double EVs.