We identified the insect iridovirus IIV-6/CrIV as a pathogen of the cricket Gryllus texensis using electron microscopy (EM) and polymerase chain reaction (PCR) analysis. Electron microscopy (EM) showed that the virus attacks the fat body, an organ important for protein production, immune function and lipid storage. During infection the fat body hypertrophied, but egg production withered, leaving the lateral oviducts empty of eggs; the females were effectively sterile. EM of the testis of infected males suggests that the testis was not invaded by the virus, although sperm taken from the spermatophores of infected males showed little or no motility. Nevertheless, males and females continued to mate when infected. In fact, infected males were quicker to court females than uninfected controls. The virus benefits from the continued sexual behaviour of its host; transmission studies show that the virus can be spread through sexual contact. Sickness behaviour, the adaptive reduction of feeding and sexual behaviour that is induced by an activated immune system, is absent in infected crickets. Total hemolymph protein is reduced, as is phenoloxidase activity, suggesting a reduction in immune protein production by the fat body. The evidence suggests that during IIV-6/CrIV infection, the immune signal(s) that induce sickness behaviour are absent. Curtailment of a host's sickness behaviour may be necessary for any pathogen that is spread by host sexual behaviour.