Yeah, Ike is correct... but that has nothing to do with Pokerus... 
Leveling a Pokemon even after its pre-evolved form will not lower its stats, and most of the time, you even learn some new moves quicker than evolved. 
When a Pokemon evolves, the game will recalculate the stats regardless of level, and according to the evolved Pokemon's base stats. 
Like how Magikarp's base Speed is 80, and evolved as a Gyarados is 81.
The game will recalculate the stats according to that.
(Bad example... but l hope you get the point?)  
If you have a Lv 100 Feebas, and a Lv 100 Milotic, and both have perfect IVs, with max EVs in Speed, when evolving the Feebas, it will have the same exact Speed stat as the Lv 100 Milotic.