The exact cause of GBS is not known. Researchers don't know why it strikes some people and not others. It is not contagious or inherited.
What they do know is that the affected person's immune system begins to attack the body itself. It is thought that, at least in some cases, this immune attack is initiated to fight an infection and that some chemicals on infecting bacteria and viruses resemble those on nerve cells, which, in turn, also become targets of attack. Since the body's own immune system does the damage, GBS is called an autoimmune disease. Normally the immune system uses antibodies (molecules produced in an immune response) and special white blood cells to protect us by attacking infecting microorganisms (bacteria and viruses). In Guillain-Barre syndrome, however, the immune system mistakenly attacks the healthy nerves.
Most cases usually start a few days or weeks following a respiratory or gastrointestinal viral infection. Occasionally surgery will trigger the syndrome. In rare cases vaccinations may increase the risk of GBS. Recently, some countries worldwide reported an increased incidence of GBS following infection with the Zika virus.