Remus was just five years old when he was inflicted by Lycanthropy.

His father, Lyall Lupin, had once encountered the werewolf Fenrir Greyback and had said that he deserved nothing but death. This was because Greyback used his lycanthropy in an adverse way inflicting children and creating an illegal werewolf army.

Greyback decided to get back on Lyall by inflicting his son by the same curse.

So one night when five year old Remus was asleep in his bedroom Greyback forced his way in through the window and bit the poor child. And thus, Remus too became a werewolf.