As Lynx-Lynx noted, this really isn’t a novel. Although it does follow several characters, the writer isn’t primarily interested in the development of the characters, but in how a community reacts to operating under extreme conditions like loss of liberty, separation from loved ones, and the high risk of death.
As noted, there are times when Camus gets up on a soapbox to rant, as during Tarrou’s diatribe on capital punishment. There were times reading this when I almost fell asleep, and other times I was amazed by the prose -
"During all the late summer and throughout the autumn there could daily be seen moving along the road skirting the cliffs above the sea a strange procession of passengerless streetcars swaying against the skyline. The residents in this area soon learned what was going on. And though the cliffs were patrolled day and night, little groups of people contrived to thread their way unseen between the rocks and would toss flowers into the open trailers as the cars went by. And in the warm darkness of the summer nights the cars could be heard clanking on their way, laden with flowers and corpses"
It was the best of books; it was the worst of books
I suspect that the main theme of The Plague is the experience of occupied Europe under Nazi rule, and of the responses to that occupation. Camus published The Plague in 1947, after having lived through the Occupation, and having been the editor of the underground Resistance newspaper Combat - was this the equivalent of the heightened risk of being on a sanitary team within an already dangerous environment? There are many echoes of the Occupation in The Plague, for example:
“What with the gunshots echoing at the gates, the punctual thuds of rubber stamps marking the rhythm of lives and deaths, the files and fires, the panics and formalities, all alike were pledged to an ugly but recorded death”
as well as other details like the stadium internment camp within the universal prison of the city. There is the odd little moment of a visit to the Place d’Armes where “Gray with dust, the palms and fig trees drooped despondently around a statue of the Republic, which too was coated with grime and dust” - a reference to the Vichy regime and the tarnished end of the Third Republic?
There is also the theme of the Absurd, and how individuals respond to the challenge: physical suicide (Cottard), philosophical suicide (Fr. Paneloux), or the recognition of the absurd condition and death and the human struggle against them (Rieux, Rambert, Grand and Tarrou)