When my PN started full force in 2016, I was referred to Duke who did all the standard Mayo recommend tests. Everything was negative except for my EMG which showed severe axonal sensorimotor PN. The doctor’s diagnosis was that it was Idiopathic PN “possibly from a Guillain Barre variant or virus”. I was negative for Guillain Barre itself, so that statement from Duke leads me to think the medical field must think other viruses unable to be tested could trigger PN. That’s how I interpreted that, but I’m not a medical person and can’t be sure.