A 1976 swine flu vaccine was associated with guillain-barre syndrome (pronounced ghee-YAN bah-RAY), in which the body damages its own nerve cells, causing weakness and sometimes paralysis.

No one is entirely sure why the vaccine caused guillain-barre syndrome, but it affected more than 500 people and killed about 25. (the feared swine flu epidemic never materialized that year.)

Vaccination is not without risk in itself, and pandemic flu vaccines have been associated with a rare neurological condition known as Guillain Barre Syndrome (GBS).