Camus actively rejected labeling his work as “existentialism”. It only frequently gets lumped into that category because of his relationship/friendship with Sartre and de Beauvoir and because some of his work contains existentialist-like themes. He was more accepting of the term “absurdism” for his work. But his work never fully aligned with existentialist thought.

“The Plague” is typically read as an allegory of the French resistance to the Nazi invasion and occupation of France, and an analysis of the relationship that humans have with authoritarianism, fascism, and how we can either help or hurt each other in times of crisis. Camus was also a humanist and believed that people were more good than evil (which he comments on in “The Plague”).

Source: I got my university degree in Philosophy and Camus was my main subject of interest/study for a number of years (and continues to be even now).