The narrator in "The Yellow Wallpaper" is likely suffering from depression and likely from postpartum psychosis (at least in part) because of the young baby mentioned in the story. She finds that she cannot take care of her baby and has no desire to be near him, as his presence makes her "nervous."
The specific illness is never mentioned, but the narrator does relate that her husband suggests: taking her to Dr. Silas Weir Mitchell, a doctor who became known for working with intellectual women suffering from similar symptoms and prescribing a "rest cure."
Although she longs to do more in the beginning of the story than simply rest inside her room, her husband feels that he knows what is best for her (and even what ails her) and insists that she rest. He calls her his "little girl" and refuses to listen to his wife's concerns about her health. She expresses feelings that she isn't getting better, that she isn't gaining weight, and that her mind still isn't clear. He replies by telling her that she simply can't see the truth of how much she's improved.
The narrator is surrounded by men who claim to know more about her condition than she does. Both her husband and her brother are physicians who insist that she has simply a "temporary" nervous depression. The specialist they consider sending her to is also a male. So the narrator suffers, very much alone, and even becomes afraid of her husband before her complete break with reality in the final scenes of the story.
There are two core issues with the narrator, as evidenced through the events of the story. First, she suffers from some mental health issues and isn't receiving the treatment she needs. Second, her own concerns about her health are dismissed and trivialized by others until she becomes completely trapped in her own mental world and suffers a psychotic break.