What causes autoimmune diseases?

It is still unknown why the immune system becomes misdirected. However, there are several factors which may influence the development of an autoimmune disease. For instance, a genetic predisposition is mandatory.

Although autoimmune diseases are not genetic diseases, a genetic predisposition is mandatory for the development of such a disease. This means that certain combinations of genes lead to a higher risk to develop an autoimmune disease than others.

In addition to genetic predisposition, different environmental factors are necessary. They are the trigger to finally turn the immune system on. 
Infections, vaccinations or immunizations, hormonal changes, smoking or nutritional deficiencies besides others are considered as environmental factors which may provoke an autoimmune reaction.

Autoimmune diseases cannot be caught from another person.