Correcting for all risk factors, no pair of viruses involving the Feline Immunodeficiency Virus (FIV-FHV, FIV-FCV, FIV-FPV) was significantly associated. This result is at first surprising because, as in humans infected by HIV, feline AIDS is characterised by a chronic immunodeficiency, allowing subsequent opportunistic infections (review in [84] ). Indeed, although FIV positive cats can mount immune responses to administered antigens other than during the terminal phase of infection, their primary immune responses may be delayed or diminished [85, 86] . Experimental studies also revealed that cats co-infected by FIV and FCV or FHV had more severe disease signs than non-FIV infected cats [87, 88] . In addition, the presence of FHV was shown to accelerate FIV transcription through the activation of the FIV long terminal repeat [89] , a phenomenon that was also shown in vitro for the human versions of the viruses, HSV2 and HIV [90] [91] [92] [93] . Those laboratory experiments show that FIV infection may increase the severity of FHV or FCV-induced clinical signs but do not address the question of the effect of FIV on the sensitivity to FHV or FCV infection. Furthermore, the few epidemiological studies interested in the question did not demonstrate any epidemiological association between FIV and FHV [94] . In other words, if experimental investigations suggest a synergy between FIV and FHV and between FIV and FCV towards a more severe disease, our sero-epidemiological study suggests that the identified risk factors explain by themselves the apparent increase of double sero-positive individuals.