We are never told exactly what Rieux’s young wife dies of, but her departure for a sanatorium at the very start of the story suggests that maybe she is suffering from tuberculosis or some similar ailment. It was common practice in the earlier years of the twentieth century for people suffering from such illnesses to remove to sanatoriums.
It is unlikely that Rieux’s wife would have contracted plague in the clinically clean conditions of a sanatorium, and besides this particular outbreak is shown to be more or less confined to the town of Oran, which she has left behind. Another factor that makes it unlikely to be plague is that the plague victims usually die within a matter of days, whereas Rieux’s wife dies about ten months after leaving for the sanatorium. We can conclude that she probably does die of tuberculosis or some similar disease, and not plague.
Rieux’s wife hardly appears in the novel at all, yet she and her illness are significant. Although she is probably not suffering from plague herself, she does seem connected with it. She leaves Oran just as the plague is beginning; news of her death comes when the plague is finally over. And consider Rieux’s reaction in the quote below, when he sees one of the first infected rats coming towards him, in its death agonies.
He wasn’t thinking about the rat. That glimpse of spurting blood had switched his thoughts back to something that had been on his mind all day. His wife, who had been ill for a year now, was due to leave next day for a sanatorium in the mountains (Part 1, chapter 1)
The infected rat immediately puts Rieux in mind of his sick wife, before the word plague is even mentioned.
The suffering of Rieux's wife, then, is linked with the suffering of the people in the plague-ridden town of Oran, but in both life and death she herself maintains a low profile. She is quiet, meek, and like her illness she is not even given a name. It can be said that she is a symbol of patient endurance; her illness and death are obscured by the more obvious hardships of the plague victims, but she is no less important. Her death affects Rieux the most, as one would expect, but he takes it stoically, just as he faced up to the plague.