The letter that Florentino finds on his doorstep is a fairly long and passionate letter filled with rage and hatred. She writes it as the final step in her attempt to win back her life, which she seemed to lose with Dr. Urbino's death. She has cleansed the house of everything that belonged to him or reminds her of him, turning his study into a sewing room for herself. She finally makes her house the clutter-free and open space she always wanted. At first it did no good, but she slowly started to feel better except for the bitter memory of Florentino hanging over her head. This is why she writes the letter.
During this time Florentino has been sleepless and depressed. He tells America Vicuna that he is going to marry, but she assumes he is joking. The next day he gets Fermina's hate letter. He is not upset by the content of the letter because he had rather expected it, and the fact that she has written him at all gives him the opportunity to respond. He learns to type on a typewriter for the purpose of his response. It takes him twelve days to complete a letter successfully, and the letter he writes is unlike any he has ever written--closer in logic and tone to a business letter than any of the business letters he had tried to write.
The letter does not refer to the past at all. Instead it is an extensive meditation on life, aging, and love. He does not expect a reply. He becomes very excited as days pass and the letter is not returned to him. He continues to write letters, slowly at first because his typing is poor, but eventually one letter every day. He anticipates a favorable reply eventually. Thus he begins another renovation of his house so that it will be fit for Fermina. He still sleeps with two of his former lovers every once in a while, but he has trouble with America, who feels devastated by his rejection of her.
On the first anniversary of Dr. Urbino's death, Fermina and her family hold a memorial Mass. Florentino is not invited, but he attends anyway. After the Mass, Fermina thanks all of her friends individually for coming, including Florentino, whom she thanks graciously. She read all of his letters with great interest, and in fact she found that they helped her desire to keep living. Afterward Florentino writes her a letter of thanks for her kind greeting, then nothing else for two weeks.
He shows up at her door. Florentino expects to be turned away, but he is not. As he waits for Fermina to come down, his body gets the better of him, and when she finally arrives he must leave after only a few words so that he does not soil himself in front of her. He returns two days later to have coffee with Fermina. They both feel uncomfortable. As Florentino leaves, however, Fermina invites him to return whenever he likes. He returns a few days letter. They discuss his letters and how they have helped Fermina.
Their visits gradually become weekly and part of the patterns of their lives. Whenever Florentino tries to push Fermina to talk about their past, she rebuffs him. Dr. Urbino Daza and his wife join them for cards at times, and they all enjoy the games together. Urbino Daza invites Florentino to meet him for lunch, where he thanks Florentino for his friendship with his mother, stressing the importance of the elderly taking care of each other so that no one else needs to bother. Florentino is not offended, because he sees it as a good omen of the time when he will ask Dr. Urbino Daza for his mother's hand.
Florentino in fact is so happy after this meeting that he trips on a staircase and sprains his ankle. The doctor confines him to bed for sixty days. Leona and America both take care of him, and they both feel shocked that he never tries to seduce either of them. America starts to do badly at school and becomes very unhappy.
Since his visits with Fermina are interrupted, they start a new mail correspondence. Their letters become very personal, but Fermina still rebuffs any attempt at romance or nostalgia that Florentino puts forward. She is surprised at how lonely and bored she feels without their Tuesday meetings.
During this time a newspaper called Justice publishes an article accusing Lucrecia del Real and Dr. Juvenal Urbino of having had an affair. It is not true, but Fermina believes it. The paper also publishes a true article on the illegal dealings that Lorenzo Daza used to make his fortune, which upsets Fermina greatly. When Florentino is well enough to go to see her again, he finds her completely different, seeming much older and much more unhappy as a result of the two articles. With Florentino's return, Fermina starts to do better, but her daughter Ofelia hears about their relationship and comes to put an end to it. Fermina will not listen. She instead kicks Ofelia out of her house permanently for her cruelty.
Upset by her daughter in addition to the two articles in Justice, Fermina decides to take Florentino up on his offer to go on a riverboat trip. At first she is depressed. She invites Florentino to sit with her on her private deck, where he reminisces while she sits quietly, smokes cigarettes, and cries. They hold hands. Although they do not kiss goodnight, the next morning when Fermina reads the letter he has left, her heart races like a teenager's. When she meets him on the deck, she sees that he has changed his usual outfit for perhaps the first time in his life.
That night Fermina allows Florentino to kiss her. Slowly they get closer and closer. One day on the boat, Florentino receives a telegram that America Vicuna has committed suicide. He is greatly disturbed and upset, but he does his best to avoid thinking about it. The boat runs out of fuel for a week, and they are stuck in terrible heat. This calamity increases their love, for the heat makes it easier to love without questions or embarrassment. They finally make love. The first two times do not go well, but they slow down and take their time, and they finally find each other in the right way.
When they reach the last stop of the boat, Fermina sees people she knows boarding. She feels horrified, for she does not want anyone to see the Widow Urbino on a pleasure cruise. Florentino talks to the Captain, and they decide to fly the yellow flag that warns of cholera so that they can ride back without any other passengers. The Captain brings his own lover on board, and they are all completely happy. As they get closer to home, Fermina and Florentino both dread leaving the ship. The Captain cannot figure out how to deal with the officials who insist on inspecting the boat's passengers, so together they all decide to keep sailing with the yellow flag flying "forever."
In this final section of Love in the Time of Cholera, writing letters again becomes very important. It is through his letters that Florentino finally reaches Fermina's heart. These letters are, interestingly, unlike any he has ever written. The two become close through their Tuesday meetings as well, but for a year before, and then the two months of Florentino's convalescence, letters are their only communication. As in the second section of the book, the reader does not see any of these letters. This withholding seems especially significant because the letters Florentino writes deal with life, love, aging, and death, four of the most important themes throughout the novel.
By not allowing the reader to see Florentino's meditations on these issues, Marquez may be suggesting that an answer in words is not possible or complete, whether the words are in Florentino's letters or in his own novel. Understanding these issues, perhaps, requires experience in a lived life, and no secondary account of it can do it the proper justice. Fermina does, however, find deep solace from these letters, for these meditations directly involve her own life, so Florentino's writing, although it is invisible to the reader, is not worthless for the writer or the recipient.
Fermina's character develops more in this section than it has in any other part of the book. Through five sections and fifty years of marriage she was stubborn, proud, and haughty. Now, Marquez finally portrays the softer, more complex, and slightly less sure characteristics of Fermina.
That the evolution of Fermina's character comes with Dr. Urbino's death is not surprising. The first days of her widowhood seem most alarming to her because she has no identity without her husband; her entire life has been based around him. It is only with his death that she is given room to blossom fully into herself. This is why it would seem inappropriate to marry Florentino at the end of the book; some other solution is necessary. Spending the ends of their lives together on a riverboat with no destination offers her much more freedom to develop a free identity--even while in the throes of love.
This freedom and much additional freedom is allowed because of a false signal of cholera, the archetypal illness of the novel. The yellow flag allows Fermina and Florentino to avoid the world of social mores that waits on any shore, for people are happy to leave sickness, unlike love, isolated. Like Dr. Urbino, who used medicine as a cloak under which to visit Barbara, Fermina and Florentino can wave their flag and keep everyone from suspicion. Sickness and aging tend to excuse people from the normal social rules, allowing people who are old or sick to live with a freedom that the healthy and young rarely have. In Marquez's novel, love and sickness are conflated repeatedly, and a love like Fermina's and Florentino's, like a sickness, does not follow societal rules, so it is appropriate that the social guidelines allowing the sick a certain freedom (quarantined on the ship, but free to move across the waters) also give freedom to Fermina and Florentino.