There are topics spoken of in this play that place it squarely in the naturalist tradition.  Naturalism deals with the seamy side of life.  (A helpful discussion of Naturalism in American can be found at http://www.wsu.edu/~campbelld/amlit/natural.htm --- although it's about America, the concepts of naturalism are universal.)  Topics such as venereal disease, illegitimate children, and incest would never have been spoken of in other traditions because they were not subjects for "polite" conversation. 

There are many examples of naturalist literature at the turn of the Century in Europe and America.  Since I am more familiar with American Literature, I would suggest you read Theodore Dreiser's "Sister Carrie" which has some of the same themes as "Ghosts," although there is no mention of disease.  Stephen Crane's "Maggie:  A Girl of the Streets" explores many of these same themes.