Passage: In 1920, d'Hérelle travelled to Indochina, pursuing studies of cholera and the plague, from where he returned at the end of the year. D'Hérelle, officially still an unpaid assistant, found himself without a lab; d'Hérelle later claimed this was a result of a quarrel with the assistant director of the Pasteur Institute, Albert Calmette. The biologist Edouard Pozerski had mercy on d'Herelle and lent him a stool (literally) in his laboratory. In 1921, he managed to publish a monograph, The Bacteriophage: Its Role in Immunity about his works as an official Institute publication, by tricking Calmette. During the following year, doctors and scientists across western Europe took a heightened interest in phage therapy, successfully testing it against a variety of diseases. Since bacteria become resistant against a single phage, d'Herelle suggested using "phage cocktails" containing different phage strains.

Question: When did d'Herelle travel to Indochina?
1920