We think the title is pretty self-explanatory, since the book is about a plague. Also, Camus seems to like short, bland, descriptive titles.
Yet, "the plague" means a little more than the outbreak of pestilence that goes down in Oran. The novel ultimately suggests that the citizens of Oran, and all of us, in fact, live in a constant state of plague. The justification is that 1) we may all die at any moment, 2) we all suffer all the time, and 3) we are all in a constant state of exile, though it is generally self-prescribed. The solution offered is a strict diet of self-awareness, followed by consciousness push-ups, and never-ending battle against human suffering.
And you thought you were getting off easy here, didn’t you?