In the previous study, the antigenic protein with the molecular weight of 34 kDa has been identified, purified, and characterized as cytolysin, a cytotoxic protein from the outer membrane of S. typhi. The utility of 34 kDa cytotoxin protein against the IgA antibody has been well demonstrated as a diagnostic biomarker for typhoid fever [31, 32]. Here, PRF1 is linked to typhoid fever.