Scientists are not yet sure what causes Guillain-Barré syndrome (GBS).

In more than half of all cases of GBS, minor respiratory or gastrointestinal illnesses precede GBS. Some experts believe that when these illnesses activate the immune system to defend the body, the immune system mistakenly attacks healthy nerve cells as well. This may occur when certain viruses or bacteria change the cell structure of healthy nerves in the body, so the immune system is unable to properly recognize them and treats them as if they were foreign cells. Certain pathogens may also somehow affect the immune system itself so that it is unable to recognize certain types of cells as its own.

Conditions and situations associated with the onset of GBS include:

• Bacterial infections. A variety of different bacterial infections (including Lyme disease and Hib infection) have been associated with the onset of GBS. Infection with the bacterium Campylobacter is the most commonly identified illness preceding onset of GBS. Campylobacter is often found in undercooked food, especially poultry.
  
  
• Viral infections. A wide variety of viral illnesses are associated with the onset of GBS. These include colds, the flu, mononucleosis, HIV, cytomegalovirus infections, and herpes simplex viral infections (the cause of cold sores).
  
  
• Vaccinations. In rare cases, GBS has been reported in people who recently have received certain vaccines (e.g., influenza, meningococcus, swine flu). However, this link is not clear and experts generally believe that people who receive vaccinations are at little or no added risk of contracting GBS.
  
  
• Recent surgery or childbirth. These have also been associated with GBS. GBS may also occur with certain systemic medical conditions such as lupus and Hodgkin’s disease.

GBS can strike people at any age, but is more likely to occur among teens and young adults (15 to 35) or among older adults (50 to 75). It occurs in both genders, although just slightly more frequently in men. GBS does not usually occur in young children under the age of two years. This may indicate that some maturation of the immune system is required before GBS can occur.