Guillain-Barré syndrome is the most common and most severe acute paralytic neuropathy, with about 100 000 people developing the disorder every year worldwide. Under the umbrella term of Guillain-Barré syndrome are several recognisable variants with distinct clinical and pathological features. The severe, generalised manifestation of Guillain-Barré syndrome with respiratory failure affects 20–30% of cases.
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Understanding of the infectious triggers and immunological and pathological mechanisms has advanced substantially in the past 10 years, and is guiding clinical trials investigating new treatments."

Source : The Lancet Volume 388, Issue 10045, 13–19 August 2016, Pages 717–727.

The disease causes muscle weakness which usually develops in the feet and hands, then spreading to the arms and upper body. It can be severe enough to cause breathing difficulties, and can develop over many weeks or just a few hours.

It's known to be an auto-immune problem, with the body's own immune system attacking the nerve cells of the peripheral nervous system and related support structures (details at Wikipedia)

Roughly 60% of people with Guillain–Barré syndrome have experienced an infection of some kind directly before the onset of the condition (esp.Campylobacter jejuni) but how and why the infections trigger GBS is unknown. And the remaining cases have no causal explanation as yet.

In addition, the genetic and environmental factors that affect an individual's susceptibility to develop the disease are also unknown.