Autoimmune diseases affect around 5% of the world population, particularly people from developed countries and those numbers are rising. This biased distribution is explained by existing differences in the genetic background and environmental factors. Just because you have a genetic predisposition does not mean you will develop an autoimmune disease.
Influential contributing factors play a significant role. Stress, toxic environmental chemicals, unhealthy diet choices, unhealthy relationships, bad sleep habits and a compromised microbiome known as "Leaky Gut" often caused by the overuse of antibiotics. As you can see many of these outside influences can be avoided if you know you have a genetic susceptibility to an autoimmune disease. If you don’t have a DNA marker for this scenario you can screw up as much as you want but you probably wont feel that great either.
The immune system is designed to protect us from the “Bad Stuff”; attacking Viruses, Injuries, Infections, etc. Our Immune System plays a significant role in the emotional Fight or Flight syndrome. When this protective emotion is left on the "fight mode" for extended amounts of time our immune system becomes overtaxed and eventually the switch does not turn off. This creates the perfect foundation for any of the over 100 autoimmune diseases caused by a confused immune system attacking healthy tissues.