The attack on the nerves takes place in the brain - but it apparently originates in another organ: the intestine.
Billions of microbes that help digestion adhere to its moist mucous membrane.
Immune cells are also located in the intestinal wall, and when they come into contact with bacteria, their metabolic products and food, they are calibrated to suit the human environment.
"The intestine is the Olympic training center for the immune system," says Reinhard Hohlfeld, emeritus neurologist at the Ludwig Maximilians University in Munich.
The immune system is constantly being adjusted to germs.
The training center also produces immune cells from the T-cell family that can migrate into the brain, for example when the brain is threatened by a large number of pathogens such as meningococci and streptococci and its own protective mechanisms can no longer counteract them.
Other causes - not yet known in detail - also guide the immune cells into the brain.