Here’s a link to an article in the Oxford Journals on the subject of the possible connection between Varicella-Zoster Virus (Chickenpox and Shingles) and GBS, which illustrates the difficulty of establishing causation.
Varicella-Zoster Virus: Another Trigger of Guillain-Barré Syndrome?
I developed a terrible case of shingles at the same time that my CIDP, which was on remission, came back like never before. I was completely paralized for weeks.