Typhoid fever, caused by Salmonella enterica serovar Typhi (S. Typhi), is a major health problem especially in developing countries. Vaccines against typhoid are commonly used by travelers but less so by residents of endemic areas. The researchers used single nucleotide polymorphism (SNP) typing to investigate the population structure of 372 S. Typhi isolated during a typhoid disease burden study and Vi vaccine trial in Kolkata, India.