Bipolar disorder, formerly called manic depression, is a mental illness that causes people to have episodes of severe high and low moods. People who have this illness switch from feeling overly happy and energized to feeling very sad and vice versa. Some people are very irritable and aggressive when manic. Because of the highs and the lows — or two poles of mood — the condition is referred to as “bipolar” disorder. In between episodes of mood swings, a person may experience normal moods.