The protective effects against arthritis were only observed when a pre-established Sj infection entered acute stage. DBA/1 mice were infected with Sj for 2 weeks or 7 weeks prior to immunization with collagen II (CII) in CFA and named as ESCIA mice and ASCIA mice respectively. 7 weeks Sj infection significantly reduced the severity (A), incidence (B) and clinical signs (E) of arthritis in ASCIA mice; whereas 2 weeks infection by Sj failed to decrease the severity (C), incidence (D) and clinical signs (E) of arthritis in ESCIA mice. Arthritis was scored weekly as described in Materials and Methods. Circle and square represented uninfected CIA mice and Sj infected CIA mice respectively. CIA data represented the combined results from two different age groups from multiple experiments due to their similar severity and incidence. Data in ESCIA mice were the means and the standard errors of the mean (SEM) of clinical scores from 20 individual mice combined from two separate experiments, while data in ASCIA mice were from 13 individual mice combined from three separate experiments. Significance was tested by two way ANOVA with Bonferroni multiple comparison test with *** as p < 0.001. Incidence was calculated on the ratio of the number of animals with arthritis score >= 3 on one joint over total number of mice.