When the Captain raises the yellow flag to announce to other ports that there is a case of cholera aboard, his gesture is symbolic of Florentino's complete surrender to his plague of desires, for, at long last, he has finally been consumed by Fermina's love, and has surrendered himself to it, as a sufferer of cholera would surrender to death.
When, in Chapter 5, Florentino announces that he and Leona Cassiani have "killed the tiger," he implies that they have overcome any remaining sexual tension between them, the "tiger" representing that tension. Since Florentino first meets Leona Cassiani, there is an enduring sexual tension between them, particularly because Florentino had initially mistaken Leona for a whore. However, the current of sexual electricity that runs between Leona and Florentino lessens in the years after their first meeting. Leona and Florentino "kill the tiger" with honest communication, specifically when Leona tells Florentino, with the utmost sincerity, that she has known for a long time that he is not the man she is looking for.
In many of his letters, Florentino sends Fermina a white camellia, the "flower of promise," a gesture which represents his undying love for her. In Chapter 1, Fermina refuses the first camellia Florentino gives her from his lapel, and returns the subsequent camellias he sends her. In her refusal to accept the camellias, Fermina rejects any commitment to Florentino and his offering of love. She does not want to be bound to him, and by refusing the "flowers of promise," not only does she resist any obligation to her lover, but, as she understands it, helps to curb any thoughts of marriage that Florentino may have. Thus, his marriage proposal comes as a complete shock, and leaves her panic-stricken, for she is not yet mature enough to undertake such an immense responsibility as marriage.