Here is his story compared to Erik's, summed up (from bookstove.com) :
He might have been based on Joseph Merrick, a disfigured sideshow performer. Its not entirely clear which disease he had, it might have been a combination of Proteus
and Neurofibromatosis type I. Just as Eric in the novel, Joseph used a mask and worked at shows where he sometimes would be shown as some kind of “curiosity exhibition”. It´s said that when Alexanda of Denmark once visited him she touched his hand and Jospeh almost got a heart attack, for he never thought someone would bear to touch him. This might have inspired the scene where Christine kisses the Phantom.