Percent survival of mice following challenge with RD8 mutants. Kaplan-Meier plot illustrating survival of mice infected with wild type and RD8 operon deficient mutants. Cohorts of 11–12 mice were infected with wild type and the RD8 mutants. Survival was recorded over a 10 day period. Statistical analysis was performed using a Gehan-Breslow statistic analysis for survival.