The world health organization defines health as “the state of physical, psychological (mental), social well-being and not just the absence of disease.” When we talk about mental well-being, then, we talk about how well we feel about such things as happiness, peace of mind, ability to meet the demands of life, relationships with others, joy, satisfaction with ourselves and others (especially our families and social associates), our desires, ideas, ambitions and other similar matters. Mental health, then, is much more than just being free of mental disorders.
People have a mental illness, if there is a departure from mental well-being severe enough to interfere with their mental health. Mental disorders can be caused by physical diseases, such as infection of the brain by parasites (e.g. Malaria), viruses (such as that which causes AIDS), and bacteria. It can also be caused by physical accidents or through assault, which involves the head. Cancer of the brain and a stroke (i.e., a rupture or a clot in the brain) can lead to destruction of brain cells. There are also certain generalized diseases of the body that can have adverse effects on the brain, such as uncontrolled diabetes and diseases of the kidney, the liver, and other bodily organs.
All the above physical conditions lead to diseases of the brain generally referred to as organic diseases of the brain. However, there is a wide range of mental disorders that do not have discernible physical causes, only chemical changes that do not necessarily physically injure the brain cells. These are called functional mental disorders. Other causes of mental disorders include social and emotional stressors, depending on the type of mental health disorder.
Source: Africa Mental Health Foundation