According to statistics, about 1 in 5 adults experience some sort of mental illness at some point in their lives. It is 1 out of 25 people who experience serious mental illness. Mental illness is considered serious when the person is unable to go through with his usual routine because of his mental condition.
For example, people who are suffering from obsessive-compulsive disorder may find it hard to not follow the routine that they usually do. If something out of the routine occurs, they would like to do it again. It will make them late for the events that they have to go attend or paranoid about the things that they will encounter throughout the day.