Clinical depression is the more-severe form of depression, also known as major depression or major depressive disorder. This can be explained as an intensified version of general depression.
A person suffering from clinical depression experiences fatigue, lack of concentration, sleep disturbances, worthlessness, psychosomatic symptoms, irritability, withdrawal and loneliness.
Neurotransmitters associated with clinical depression are norepinephrine, dopamine and serotonin. Norepinephrine is involved in dealing with stress while dopamine and serotonin are involved in mood and emotion one feels.
There can be a partial hereditary link to depression. Parents, children, siblings of people having severe depression are more likely to have depression compared to the general population.
Some medications which are prescribed against some pathology, might surely heal that particular area in the body but depression can be a side-effect or extreme reaction of the same.
Chronic illnesses like HIV, arthritis, heavy diabetes, heart or kidney problems, might develop depression as the treatment can get overwhelming and frustrating after a long period of struggling.