[The hypothesis of infectious etiology for idiopathic nervous system diseases: from the postulates of Koch to the criteria of Hill].

The evaluation of the hypothesis of an infectious etiology to some neurological diseases comprises four different situations. First, numerous neurological diseases have an obvious infectious etiology (encephilitis, meningoencephilitis). Second, some neurological disorders were primarily suspected to be have an infectious etiology, but the causative microorganism was discovered either longtime after the princeps description of the disease (neurologic Whipple disease, due to Tropheryma whippelii), or at the same time (tropical spastic para-paresis secondary to HTLV-I infection). Third, for other neurological diseases, an infectious etiology that was not suspected at time of their anatomoclinic descriptions, was further demonstrated in the context of a generally complex physiopathology (Guillain-Barré syndrome and infection by Campylobacter jejuni). Finally, some idiopathic neurological diseases could be related to well known or yet unknown microorganisms, in association with some environmental factors, and with a particular genetic or acquired susceptibility of the host. The evaluation of an infectious etiology to these idiopathic neurological disorders must be envisioned according to 3 possibilities: 1) generally, the neurological disease is well defined, but its etiology remains unknown and an infectious hypothesis could be relevant (multiple sclerosis, post-polio syndrome, amyotrophic lateral sclerosis); 2) sometimes, a microorganism that is not associated with a known disease, and then qualified as "orphelin", could be associated with neurological disorders (spumaretrovirus); 3) finally, a new neurological disease could be associated with a known or yet unknown microorganism, directly or indirectly. In conclusion, some idiopathic neurological diseases could have an infectious etiology, with physiopathologic, diagnostic, prophylactic (vaccination) and therapeutic (use of anti-infectious drugs) consequences.