In all cases of Rickettsia initially suspected to be leptospirosis, typhoid fever, chikungunya, or dengue fever, diagnostic assays for those pathogens at the reference laboratory were negative, except in one R. felis case where leptospira PCR was positive and leptospira IgM and IgG sero-converted, suggesting co-infection. The gene discussed is CD40LG; the disease is coinfection.