The word "auto" is short for "automobile" (car). As a prefix, it means "by or within itself" so that we have words such as autobiography (written by the subject) and autoimmune diseases, where the body's own defenses attack its tissues.
A heart attack is an obstruction to the blood supply to the heart's own tissues. In the brain this is a stroke and in the lungs it is a pulmonary embolism. Depending on the severity of the blockage, and how long before treated, the tissues without blood supply will die.
With rheumatoid arthritis, an autoimmune disorder, the immune system starts to attack synovia, the fluid that surrounds joints. As a result joints become swollen, stiff and uncomfortable. Rheumatoid arthritis is a chronic condition, but there can be periods of relapse and relief.