How was phantom of the opera disfigured?

In the novel The Phantom of the Opera by Gaston Leroux, the main character, Erik, known as the Phantom, is described as having a hideous deformity. It is never explicitly stated what the deformity is, but several descriptions are given throughout the novel. It is said that Erik's face is "hideously scarred", and that he wears a mask to conceal it. He is also said to have a "frightful face", and a "disfigured visage". Some readers believe that Erik's deformity is the result of a childhood accident, while others believe it is a birth defect.