Later This site tells the story of the history of the universe. Click Earlier and Later to follow the story. When the immune system goes wrong it is called immune deficiency.
Aids is acquired immune deficiency, caused by a virus .
Sometimes the immune system attacks the animal it belongs to. This causes an autoimmune disease, such as rheumatoid arthritis, juvenile diabetes and perhaps multiple sclerosis. Sometimes the immune system attacks harmless things such as pollen, causing hay fever, or perhaps dust, causing asthma.