Monthly isolation profile of the Vibrio cholerae O1 and O139 serogroups from patients hospitalized with acute secretory diarrhea at the Infectious Diseases Hospital, Calcutta, India, from March 1992 to December 1998. Arrows denote the month in which changes in V. cholerae strains were noted: A, appearance of V. cholerae O139 in Calcutta in November 1992; B, displacement of V. cholerae O1 by V. cholerae O139 in January 1993; C, reappearance of V. cholerae O1; D, domination of V. cholerae O1 over