In the opening stage directions, Ibsen establishes a big wall of glass through which a "gloomy fjord landscape" is visible. The rain never stops. It particularly oppresses Oswald, who complains that it keeps him from thinking and walking. He drinks in order to deal with it.
What's all this rain about? Of course it adds theatrically to the grim atmosphere of the play and gives a lighting designer something fun to play with. But it also could be interpreted as a symbolic expression of the oppressive atmosphere Mrs. Alving has created. In her iron willed determination to bury her husband's memory, she doesn't want truth – traditionally represented by the sun – anywhere near this house. And isn't it interesting that once she accepts and acknowledges the truth of her life with her husband and her son, the sun breaks into the room.
The round table in the living room becomes a field of slaughter littered with evidence of all the battles in the play. It holds the books that symbolize Mrs. Alving's new ideas, the Orphanage papers that represent the enormous lie of Captain Alving, and the champagne that Oswald requests (a symbol of the joy of life). It is also the resting place of the lamp, the artificial light Mrs. Alving gives Oswald when he complains of the dark. What Oswald really needs is the sun (i.e., the truth). He gets it. This happens just as he slips into delirium and Mrs. Alving turns off the lamp.
When Oswald confesses to his mom that he has syphilis, he echoes the doctor's biblical diagnosis that "The sins of the fathers are visited upon the children" (2.270). While you can't get syphilis from your father (unless you sleep with him), the symbol is what's important here. Syphilis does represent what Oswald inherited from his father, but it's not really "sin."
Oswald inherited his father's buoyant, joyful nature, a nature suffocated and defiled by the puritanical Norwegian society. In this play, syphilis is a symbol of what happens when an important, natural life force (i.e., sex) is driven out of "respectable homes" and into dark corners like the brothel Engstrand wants to start. Mrs. Alving is a main player in forcing her husband into a twisted expression of his passion. His infection comes back to haunt her, as she faces the decision of whether or not to kill her diseased son.