Depression is one of the most common illnesses in the United States. Approximately 21 million people are affected by depression each year and it is one of the leading causes of disability for individuals between the ages of 15 and 44 (CDC).

Researchers believe that depression is a result of an imbalance of naturally occurring chemicals (neurotransmitters) produced in our bodies called serotonin and norepinephrine (Meyers, 2000). These neurotransmitters, also known as "chemical messengers," help nerve cells communicate with one another by sending and receiving messages. These messengers are believed to regulate individuals' overall mood and pain receptors. Vaidya & Duman (2001) further explain that the regulation of intracellular messenger cascades exerts a powerful control on almost all aspects of neuronal function, inclusive of neuronal morphology, gene expression, activity, and survival. Those who experience depression are believed to have a low supply of chemical messengers, thus resulting in depressive symptoms. Meyers (2000) states that the synthesis of most neurotransmitters is controlled within the brain. For some neurotransmitters, the amount of biochemical precursors present in the brain can influence their rate of synthesis.

Depression is also a common secondary condition. It is often times a comorbidity of other chronic illnesses and diseases such as diabetes, heart disease, obesity, spinal cord injuries, multiple sclerosis, stroke, Parkinson's disease, and arthritis. Depression can also worsen or exacerbate an individual's current condition.