IFNG and infection: O. tsutsugamushi-specific IFN-γ responses were observed post infection from all infected macaques compared to uninfected macaques, with a 20-fold (mean 235, 95% CI 200–270 SFC/106 PBMC) increase at Day 7, a 100-fold increase at Day 14 and maintenance of high level responses to the end of the study (Day 28).