The rats make the citizens of Oran nervous, which makes us nervous, which means this initial situation isn’t messing around.
The conflict comes when the plague transfers from the rats to humans. Generally, any pestilence outbreak is going to constitute a major conflict. Chances are, this conflict is going to be followed by a complication. Maybe even a climax.
The plague is here and everyone knows it. There is no easy way to fix it. This is the complication because now in addition to the presence of a killer disease, people are trapped with it inside the city. It is their problem, and their problem alone. Also, the disease gets worse (now it’s in a deadlier and more contagious strain).
Jacques, an innocent boy, dies from the plague over the course of many torturous pages while grown men stand by and weep, powerless to help. This is clearly the emotional climax of the play for many of our main characters. Rieux has an outburst and even Paneloux – maybe – begins to doubt his faith (though that is subject to debate).
Why is Grand’s illness the suspense stage? Because, unlikely as it may seem, Grand is identified as the would-be hero of Oran. His death might not have heavy practical repercussions, but it would sure have some serious symbolic ones.
Tarrou, who has gotten as close to prophesying his death as a man writing a journal can, dies expectedly. That’s why it counts as denouement and not suspense. (Also, the narrator’s been reading his journal for 250 pages, which is a pretty good sign that Tarrou died and had his journal passed on to some other, non-dead person.) Grand’s gradual recovery of course signals the end of suspense and the start of the end, and Cottard’s madness is just the wrap-up of our one remaining lose end. No, wait, the narrator revealing his identity is the one remaining lose end. So we get that out of the way, too. (You guessed it, the narrator is Rieux.)
The conclusion consists of the narrator explaining his reasons for telling us this narrative. We are left to ponder these in conclusive glory.