Guillain-Barré syndrome is thought to be caused by a problem with the immune system, the body's natural defence against illness and infection.
Normally the immune system attacks any germs that get into the body. But in people with Guillain-Barré syndrome, something goes wrong and it mistakenly attacks the nerves.
This damages the nerves and stops signals from the brain travelling along them properly, which can cause problems such as numbness, weakness and pain in the limbs.
It's not clear exactly why this happens. The condition isn't passed from person to person and isn't inherited.
Sometimes Guillain-Barré syndrome appears to have a particular trigger. Some of the main triggers associated with it are outlined below.
In about two in every three cases, Guillain-Barré syndrome occurs a few days or weeks after an infection.
Infections that have been known to trigger the condition include:
In the past, vaccinations (particularly the flu vaccine used in the US during a swine flu outbreak in 1976) were linked to an increased risk of Guillain-Barré syndrome.
But research has since found the chances of developing the condition after having a vaccination are extremely small.
For example, a study into the vaccine used during the 2009 swine flu outbreak found that for every million people who had the vaccination, there were less than two extra cases of Guillain-Barré syndrome.
The benefits of vaccination are likely to outweigh any potential risk, as infections such as flu are more common triggers of the condition.
Other possible triggers for Guillain-Barré syndrome include: