Among the 5 acute sera with negative ELISA, one was from a patient who had 4-fold IFA titer increase between acute and convalescent sera, one patient was PCR positive in acute serum and 4-fold titer increase, one was PCR positive in acute serum only, one was positive because the IgG or IgM titer was ≥400 in convalescent serum, and one patient who was diagnosed as ST positive based on a 4-fold titer increase also showed 4-fold increase in IFA titer for leptospirosis, suggesting that it is a co-infection of ST and leptospirosis. Here, CD40LG is linked to coinfection.