Chickenpox is one of the most concerning viruses in people that are immunosuppressed especially because of things that affect lymphocytes (especially steroids). The worry is that the chickenpox can spread throughout the body including to the liver (causing liver problems) and to the lungs (causing breathing problems) or even to the brain. Normally people contain chickenpox that returns to one area of skin as shingles (herpes zoster), but if someone is immunocompromised, the chickenpox can disseminate outside of that area and to other parts of the body. About 100 people a year in the US die of the chickenpox virus. The first child I ever helped to care for that died, died of chickenpox, so I have always treated this disease with respect. I do not mention dying to be scarey, but to say that this can be serious. I work with children with cancer and we are very cautious about chickenpox because of this. we recommend that people stay away from anyone with chickenpox. Usually healthy people are safe around people with shingles if this is covered, but we tell our families to not let the immunosuppressed children be around people with shingles because it is not worth the risk or possibly getting chickenpox/shingles and having to get treatment for this.
I have helped to take care of one child who had chickenpox (not shingles)four times, so it is not true that you cannot get it again–it is just incredibly rare and the person has to not have a normal immune system. The most common reason for someone to get chickenpox a second time is the fact that someone had it as an infant less than a year old and did not get protective antibodies.
Any infection can cause a flair in the immune system and potentially cause worsening of GBS/CIDP, but I think the bigger worry is the fact that chickenpox can be really serious in itself especially for people on immunosuppressants that affect lymphocytes.
WithHope for cure of these diseases.